I cannot recommend this book enough.
https://www.amazon.com/Burn-Fat-Feed-Muscle-Transform/dp/0804137846
Read Burn the Fat Feed the Muscle and it will answer all your questions and more. I bought the audio version and the hardback to refer back to.
I’m reading Burn the Fat Feed the Muscle right now and it’d be all you’d ever need.
Buy this book read it, read the chapter in carb cycling and that weight will melt off in 2-3 weeks.
https://www.amazon.com/dp/0804137846/ref=cm_sw_r_sms_api_r4Ktxb...
This is gonna get buried, but 120lbs in 6 months is way the hell TOO MUCH WEIGHT. This is, in effect, a starvation diet and is both extremely bad for you, and your weight will generally slam back on as you drift off of it.
Tom Venuto's book "Burn the Fat Feed the Muscle" will help you get the info you need to isolate your ideal calorie level based on activity level. FWIW, 14-16 is for a moderately active person, 12ish is ideal for a sedentary lifestyle, depending on age.
But anyway, in general a pound of fat is 3500 calories. So, if you cut 3500 calories a week from your maintenance intake, you will lose 1 pound a week. (Maintenance intake being ~14x your weight if you're active at all). So, theoretically, if you drop 7000, you'll lose 2 lbs, etc.
The thing is that you NEVER start eating at your IDEAL maintenance level, unless you've only got 10 or so pounds to lose. It's bad for your body, you'll be very hungry, you'll be lethargic, you'll be an asshole, and your body may well switch itself into starvation mode, where it starts eating muscle to feed itself because it thinks it's dying. You'll likely enter ketosis (which is also a valid diet, visit /r/keto for more info) where you'll drop weight fast but potentially cause issues for yourself. Your bowel movements and urine will be awful, possibly even painful, and you'll generally be uncomfortable all the time. You'll clot less well and have headaches.
So if you weigh 250 and you want to drop weight, you find ~80-90% maintenance level, and eat there. As you approach that weight and find your loss plateauing, you drop again, then rinse/repeat. If you do this while increasing activity level at all, your weight will fall off if you're very large. The closer you get to your target weight, the slower the loss will be, and the more exercise you'll have to do.
I'm glad the OP's brother is doing better, if you try this as a 30 year old you'll fail---or you'll wind up just as big or bigger a year later. 1400 calories a day isn't enough for anyone over about 100lbs.