My stepdad was like this too. What I wish I'd said to him: Everyone can be kind. What if you were kind and tried to help me build systems that supported my memory?
But then he probably would have smacked me.
In all seriousness, talk to your mom. Tell your mom that you need to learn how to compensate for the fact that your brain works differently than most. There's a whole world of coaching, books, and skills people with ADHD can learn to compensate. If her boyfriend cares about her, he'd get proactive and work with her and you to read some of those books with you and help you.
I'd recommend starting with The Smart but Scattered Guide to Success.
I'm a little late to the game and most of my advice has already been said, so I'll just tell you this. My son has exhibited a lot of ADHD symptoms, although we have decided to wait until he's older to formally get him diagnosed. A teacher recommended the book, Smart But Scattered which has been a huge help. It's not specifically for kids with ADHD but because many of the issues with ADHD stem from executive functioning deficits, you can get a lot from this book.
Medication facilitates sufficient capacity but it does not compensate for deficient capacity. Medication helps what works in your brain to work more efficiently, without as much strain or effort, which means doing specific tasks on medication is less exhausting than completing those same tasks without medication.
However, medication does not create capacity. Nobody expects anyone to fit 20 gallons of garbage into a 13 gallon capacity garbage bag. ADHD affects the capacity of the frontal cortex. Some frontal cortex executive skills you will do easily and well, and medication will help you to utilize those strengths with greater ease. Lesser capacities might be enhanced with medication. There is simply no amount of medication, organization, facilitation or scaffolding to support those weakest capacities to make them as viable as your strengths.
To misunderstand conditions like ADHD and think the person just hasn't tried hard enough to overcome their weaknesses is like asking why someone can't just hurdle the Grand Canyon. It's ludicrous to suggest, yet our well meaning friends and family think they can help us overcome ADHD with just the right tools. My dog and cat could horde all the spoons they want, but without a thumb, they don't have the capacity to hold that spoon, thus use it properly. That is the reality of ADHD. You can make life easier, take medication to maximize what works, find scaffolding to support what is functional and learn what to avoid to prevent frustration and overwhelm. ADHD need not be debilitating, but it will be if we constantly expect opposable thumbs to grow where there wasn't one before.
I recommend this book to better understand the the role of executive skills in ADHD:
So preferential seating (sitting them in the front) may help. There are two books I would recommend looking at. I think its good to look at a book on ADHD for kids because it will put it in language they can understand and it will put it in language you can explain to them. Which is helpful. The first is The Survival Guide for Kids with ADHD
If you have kids with executive functioning problems (constantly lose stuff, can't organize stuff, forget to turn in homework, etc), I'd suggest getting Smart but Scattered
If you are going into middle school education, there are some really great books on executive functioning that are perfect for someone who is looking to work with young adolescents. The classic is Dawson & Guare's Smart But Scattered (they have a whole line now, like Smart But Scattered for Teens and the Smart But Scattered Guide to Success, but I like the original). I also like the boringly-titled-but-surprisingly-good The Everything Parent's Guide to Executive Functioning Disorder. I think both of those resources would be great starting points for both your capstone project and your future career.
I also wanted to comment on one of the questions you asked, about why you know tons of pieces of information, and sometimes "blank" on the words you are looking for. The problem you are describing is called a "word-finding problem". Sometimes, people only have trouble finding the specific words they want; sometimes, it's a more general "rapid naming" problem, where it's hard to "pull up" all kinds of information that you know well under time pressures. Examples might include names, dates, letter/number names, automatized series (like the alphabet in order), math facts, facts you've studied for a test, foreign language words, song lyrics... basically anything you've learned through rote memorization.
Lots of people have trouble quickly and automatically "pulling up" this information. The info is absolutely 100% in their brains, but people who struggle with rapid naming have trouble "finding" the information quickly, especially under pressure (e.g., when timed; when anxious; when tired; when trying really hard to remember it). In fact, many people who describe themselves as "not a good test taker" are people who have this challenge. Rapid naming speed deficits are also implicated in dyslexia and (possibly) in ADHD.
The exact neuroanatomical correlates of rapid naming are under-studied, but the studies I have seen implicate the inferior frontal area. ADHD and other executive functioning disorders also often involve the frontal lobes (although of course this is oversimplifying). TBI also often involves the frontal lobe, either through direct (coup) injury, or indirect (countercoup) injury, so TBI can absolutely affect rapid naming. PTSD increases stress/anxiety, and stress/anxiety can exacerbate rapid naming problems because rapid naming is harder for everyone under pressure. In fact, feelings of "mind going blank" are a symptom of some anxiety disorders.
So it's all complex and interrelated. But the tl;dr is that knowing something and being able to retreive that information quickly involve separate systems in the brain, and it's actually pretty common to know something and yet have trouble retrieving that information quickly.
What an amazingly incredible and thoughtful sitter you are. The boy you are watching sounds a lot like my 7 year old son, who has many of the same issues (including push back, excuses, and exactly the same series of Lego-shower-poop-or-not vs sister vs movie that you described).
I would also describe mine as smart and energetic and kind, and although I try to be very clear as to consequences and not having power struggles and whether or not it's actually important to impose my will on something, I also recognize this as an overall pattern of disorganization on his part, of lack of prioritization, impulsiveness, and failure to predict the consequences (all of what are parts of what people call executive functioning skills).
I recommend this book: Smart But Scattered. It describes many different ways children may need help in smoothing out their lives, from chores to getting ready in the morning to doing homework to handling emotions, and gives very easy step-by-step pages for each potential issue you could want to solve.
When my youngest was 3, I made a temper management board straight from the book, with pictures of things that made him lose his temper, things he was not allowed to do (kick, throw, scream), and options he could choose to calm himself down. I put pictures of his bed, his cars, his books. The book goes into further strategies for implementing and rewarding. I used it to develop a morning routine page for each kid, with different magnets to flip up for tasks accomplished like brushing teeth and getting dressed. There are more nebulous areas of assistance, like "how to deal with disappointment," and now that I'm looking at it again, I realize that more issues have come up where I can use some concrete guidance.
In any case, aside from having these kinds of tools, definitely work hand in hand with his parents and ask for suggestions or feedback from them and see what they want or how they treat this sort of behavior. In the best case scenario, everyone will have a consistent approach.
PS -- I sometimes let the rest of the kids start or finish without the one who is not paying attention, after sufficient warnings. I do give sufficient warnings and outline the consequences, though, and fortunately it doesn't always happen to the same child all the time.
While this book is for parents with kids it has a lot of great info on exec skills and functioning, migh be helpful.
There is this one too http://www.amazon.com/Smart-but-Scattered-Teens-Executive/dp/1609182294/ref=pd_bxgy_b_img_y