Oh WOW congrats! Gish i wish I had the time to foster,I'll just have to do it vicariously through you, haha! I have a pit so hopefully I can offer some advice! Bully breeds are VERY stubborn yet very smart, they catch on quickly and are eager to please. Force will NEVER work with them (I'm not inplying you'd use force) and I dont care what other's say, they DO hold gridges, well trhey'll at least pout, haha!
Anywho your best deffense aganst boredom for your foster is exerisce! These dogs have a endless amount of energy and if left without exersice they WILL get into trouble. A long walk or strenuios play is pretty much necessary if your gonna leave him for a while in the house. Your second defense is chew toys, not those happy lil squeaky toys but hearty tough toys, cow hoofs, nyla bones, tire toys, SUPER strong, cause if he dosnt have them he WILl chew your things and it wont be his fault. I'm sure there will be a ton of other advice on here from bully owners who can give you some insight into the minds of these rascals.
I think if I ever had the time to foster I'd have to get a whole ranch cause I'd never give my foster's up, haha! But its a great thing your doing, giving this babe a great start, I wish you nothing but the best of luck!