oh Jenn -- you're in tough place and I'm so sorry you're faced with this decision.
I know Brady is young - but have you talked with him about how food effects his behavior? Not the details or the stuff that he wouldn't understand - but the basics. "when you eat X it makes your brain go into overdrive and that's when you have trouble following the rules" ............ he might be more willing to work with you in making changes if he was made a part of the process. If you truly want to go the dietary route and avoid the meds -- then I think the whole family needs to be on the same diet. Its a horrible thing for a little guy to be watching others eat ice cream and he can't have any. You could at least control the environment at home that way. The rest would be up to the other adults in his life.
That being said............. you are his mother - and you have to make this decision based on what you know you and brady and the rest of the family can handle. You're a good mom and you'll do what is best.