You are so great to take this boy into your home not to mention step up the extra notch and care enough about his health and well being!! Good for you!
I don't know if you can rent them or you have to buy one, but have you considered Wii? Sometimes places like Game Stop have them used or you can rent one over the weekend. We don't personally have one but I have considered it.
I know it sounds odd, but this system is a game system to peek the interest of boys just his age, but you have to be physically active in the game. There is no sitting idle with this video game! :)
Kids get so caught up in playing it that they never realize they are working out! They have a great deal of choices even for mommies (an aerobic pad) and all sorts of things the whole family can do together. Even your 2 1/2 year old!!!
Park further away when going to the grocery store, explain it as you don't want to get your doors banged up.
Find out what he is good at, art, drawing, computers, photography? He doesn't have to like sports but find out what he is truly good at, call the local YMCA and see if there are classes he can attend and excel.
Food choices are all about you and what you have on hand. If there is no junk then there is nothing to worry about.
Have fresh fruit, veggies chopped up in the fridge with different flavor dips, get sugar free koolaid or sugar free popsicles for treats. I cut out trans fat, cut back on sodium here and stuff and there are so many great yummy snacks we have on hand without the extra calories and nobody here even noticed. We have apples on hand always, there are baked cheesy crackers my kids love, yogurt (gogurts) my kids love and stuff like that. So if that is all there is, he will eventually just get used to that.
Don't deprive him, don't even put it on him but let him read articles about what foods are good to eat and why.
Tell him all of you will do this together so you all can live long healthy lives. Ask for his input on what he wants for himself. Good luck and God Bless.