Trend Breaking Pitbull is so gentle

This is happy. He is a rescue dog that is currently being fostered by us. He first came to us after he healed, from being attacked by the female to which he was being a brat. I don't want to jump to conclusions, but it seems fishy how he ended up here.
Fortunately for us both, is this tough as nails and he absolutely loves everything and everybody. Since coming to us, he has had to stay in his kennel most the time. But now that he's feeling better, I like to take him out during the day for a few minutes so that he can get a little bit of exercise.
The reason I have to keep him in the kennel almost all the time is that the heartworm treatment can be fatal if the patient gets too excited. That means he has to stay calm I am not jumping off the walls for 3 to 6 months. That also means I have to find ways to occupy his time or he will become bored and boredom leads to bad behavior.
I have to check myself because he's very strong which makes me be overly cautious. I know from my interaction with them so far that he is a gentle dog not aggressive at all, in fact, he didn't even fight back when he was attacked, but he is very driven when he wants to get to his Kong or toy and he's playing with at the moment. He's so big and strong, that he could hurt somebody( dog or person ) so it's important for me to be able to teach him about his own strength.
This is why I am very wary of him around the other dogs. If he was to get excited, there's a good chance that he would behave in such a way that my dog will take offense. This it would almost certainly lead to a fight.
If this happens, whether or not my dog gets hurt or it's him that gets hurt, I would be devastated. They were mean I failed at my job and I don't know if I could do it after that.