At the minimum, get a joystick or yoke. Keyboard sucks for flying. If you're interested in general aviation or airliners, start with something like this. For fighters, high-performance, or newer Airbus jets, any decent joystick will do.
If your talking about the yoke and throttle it’s the Logitech one
Here’s the link:
Logitech G PRO Flight Yoke System, Professional Simulation Yoke and Throttle Quadrant, 3 Modes, 75 Programmable Controls, Configurable Throttle Knobs, Steel Shaft, USB, PC - Black https://www.amazon.com/dp/B07QXMTNPB/ref=cm_sw_r_cp_api_glt_fabc_6474C0WSVDBFZBT84SCD?_encoding=UTF8&psc=1
Logitech G PRO Flight Yoke System, Professional Simulation Yoke and Throttle Quadrant, 3 Modes, 75 Programmable Controls, Configurable Throttle Knobs, Steel Shaft, USB, PC - Black https://www.amazon.com/dp/B07QXMTNPB/ref=cm_sw_r_cp_api_glt_fabc_H4WHMP793GH8FB4K1DFF
This says it will work for the new game on Xbox.
It sounds like the kid has a really supportive step-father. Good on you for wanting to do so much to help out.
A flight sim can be a really helpful learning aid as long as you don't expect too much from it: no sim (short of a multi-million-dollar full motion simulator) can mimic the feel of being in a real plane, but a sim can help you learn some of the basics, and MSFS can be used for very realistic VFR practice in your local area.
As far as what sort of hardware to get, it would depend very much on your wallet -- flight simulation, like flight itself, can easily eat up as much money as you're willing to throw at it. But basically if you're wanting to mimic the setup in a Cessna you'll need a reasonable quality throttle, yoke, and rudder pedals.
You can get servicable entry-level yoke and throttle from Logitech for around $170. It's not the nicest, by far, and feels kind of cheap and plasticky but it'll do the job for a while until you know if the kid is going to stick with it. Add CH rudder pedals, which aren't as nice looking at Logitech but are a not more durable for only a little more cost, and you'll have the basic controls you need for under $400.
One thing to be aware of is that a Cessna doesn't use the lever-style throttle controls that that throttle quadrant -- and most flight sim quadrants -- have. It uses Vernier-style controls, but there surprisingly aren't any really affordable sim versions of those that I know of. Lots of high-end stuff, and Saitek has a kit that's not in stock anywhere, but that's all that I know of.
If you wanted to go a little up the price scale you could look at gear from Honeycomb, which gives you really good quality for the money, but I probably wouldn't get that for a kid's first set.
There are lots of additional modules you can buy, but that would give you the most important controls in something like the form they'd be in a Cessna except for the level-style throttle.
One other thing you might want to think about investing in would be a TrackIR infrared head-tracking system. Beginning flight is all about using visual references, and being able to look around like you would in real life makes a big difference in immersion.
Screens are up to you, and if you get a head tracking system it's probably not as important to get a huge monitor, but if you wanted to get something a little bit nice that could be used for general gaming as well as office tasks, 34"-class ultrawides are now really affordable and the extra horizontal space is just about tailor-made for flying.
For your other questions... any desk chair with a lockable back will work just fine. There's no need for a tablet to start with, if he really gets into it maybe adding on that can run Garmin Pilot or Foreflight later would allow him to practice with something he's likely to actually be using in the cockpit but at the beginning it would just be a distraction.
Recommending specific sim software is a dangerous business on this forum, but the thing I can say with some degree of safety is don't spend money on Prepar3d -- it's old tech and mostly a dead end at this point. XPlane and MSFS are both great sims. My own personal opinion is that the degree to which MSFS represents the real world, as you would see it in the air, is a big bonus to a student pilot. Remember -- it's all visual at first. Being able to fly around in an accurate representation of the practice area he'd actually be flying in could be a big help.
But that's as far as I'll go in terms of concrete recommendations there to avoid having your thread derailed by fanatics one way or the other. ;)
There's a lot more that could be said, but this should give you some starting points. Again, it's great that you're so enthusiastic to help the kid out, and hopefully he'll stick with aviation. But I'd just caution you against spending too much until you're sure he's going to be at it a while. Kids are fickle, and it'd suck for you to have a bunch of barely-used hardware to offload if he decides in 6 months it's not his thing.
Good luck!
If you want the entry-level G920 experience, your only real option is probably the Logitech G PRO combo, which typically varies between $150-200 retail, so you might be able to cop it for a bit less if you can find it second hand: https://www.amazon.com/Logitech-Pro-Flight-Yoke-System-dp-B07QXMTNPB/dp/B07QXMTNPB/ref=dp_ob_title_ce
Be warned, I would say the level of that cheaper gear is below what you find in racing sims in similar pricing neighborhoods. If you're not sure if flight simming is for you, then the Logitech stuff is fine, but it will quickly feel cheapy and hollow if you take to the hobby. There's a Thrustmaster Airbus stick/throttle combo, but I think the extra two axes are extra, and obviously it's a stick. There really isn't a great option between the $250-500 price point right now if you're specifically looking for a yoke/throttle, but once you get to $500, the Honeycomb Alpha/Bravo combo is absolute butter. Worth waiting and saving for as it will provide you years and years of really quality service.
Product name: Logitech G Pro Flight Yoke System
Features:
Highest price ever: $233.71
Lowest price ever: $163.88
Average price: $176.49
Previous price: $229.86
Current price: $169.99
Last price change: -27%
Price change from average: -3%
Price change from all-time low: +3%
The item was in stock as of 06/04/2021 21:42:50 (UTC)
Selling Logitech Flight york system and Thrustmaster rudder system.
90% new! Feeling super realistic.
I use it on X-plane 11 but it also can work on other flight sims.
Flight York system + Rudder = $300 Original price is $435 and plus taxes
Text me at 347-667-9135 if you are interested. Thank you.
Here you go, OP:
https://pcpartpicker.com/list/9mTp7T
This will be an absolute monster of a flight sim PC. She'll be able to play at maximum settings with an immersive and easy on the eyes high refresh rate ultra wide monitor, and I even included a mouse that an elderly woman I worked with bought after she tried out my mouse and saw how kind it was to her arthritis. It's very ergonomic.
As for joysticks, this would likely be a great option for her:
Logitech G Pro Flight Yoke System https://www.amazon.com/dp/B07QXMTNPB/ref=cm_sw_r_cp_api_i_.yTkFbCAX0JSW
You can also buy some rudder pedals as well for added realism. There's tons of options for flight sticks and pedals and other add ons for sure.
All in all this should be everything you need to get her started.