Is their some type of ADHD community that you could ask for suggestions? They may be able to do some sort of checklist to determine what is best for your son. Bear in mind, any rough play should be avoided and your son should be part of the training process-it would help them both bond. Could he handle something like rally or agility? ( I'm sorry, I don't know much about ADHD)
I would suggest a lab-they are all around great dogs and very tolerant. It sounds like you want something smaller or I would also suggest a Newfoundland. Very VERY tolerant, patient and gentle. Ours is 8 months old, and last night DD laid nose to nose with her, with her fingers in her mouth, counting her teeth. Every once in a while, DD got a lick and would giggle. If she sees a small child/dog, she automatically lays down so they can check her out-and she does this all on her own.
Of course, she's 90 pounds, and has some serious growing to do, so that is a deterent for some, but it's all worth it.