Hi, I hardly ever answer on this post anymore because of such harsh opinions but felt that I could help you a little.
My daughter has all those diagnosis's also, and I know how hard it can be. If you ever want to talk privately send me an email.
Personally we never used a GAC for my daughter, although we had a GAC for other members in the family, like myself or my daughter, I am visually impaired and my other daughter has CP.
Anyway, I believe at some points the GAC may have helped my daughter but maybe not as much as you would think, so for what it is worth her are some of my suggestions.
You did not say how old your son is, with BP diagnosis I would assume preteen or teen. So if some of these will not work because of age just ignore them.
First you know your son better than others, so what works at home will work at the park. For my daughter that was to give her enough space not to "go off"and to know how to off set them.
So things that worked for us, eat meals either in the room, like first thing in the morning and late at night, giving her freedom from the crowds, space to relax and so on. She was in crowds all day in the park, she needed to eat quietly otherwise we found she ate tensed and moody and that would just continue all day.
Second, learn to use not only the fast pass, but also invest in something like Ride Max,so you can illiminate as much of the crowds as possible. Get to the park real early, leave at lunch time and take a nap, swim or rest, maybe eat lunch back at the room. Go back when most of the crowds are dying down.
Set up your reward system, sure he has one at home or in school. For us we bought either disney dollars or gift cards, depending on child. For every hour without a fit or fight or hurting, annoying or running away you get a dollar, by lunch time she would have enough for her own treat, which we would have bought anyway if she was good, but this was an hourly reminder of her having to work for it. Same when you go back in the afternoon.
Another thing was to bring the little annoying thing that seems to calm them, for our daughter things that clicked, like a pen. Set down the rules no poking anyone and such but then even though she may be annoying you and maybe everyone around you in line, it is a lot better than her poking others, fighting with sister, being moody and rude, walking out of line after 10 minutes forcing everyone else to go after her. To me the little annoying habit is better than the alternative, and although you may get a lot of looks, to me it is no worse than parents letting children hang on the chains or sit on the fences when in line.
I hope this helps. I don't think a GAC will be all that helpful, because of our daughter, even though we had a GAC pass we usually did not use the one for CP because the lines were usually much longer, and usually in very crowded or noisy places, or dark and boring places, which would just make our other daughter go off. We did use the pass for both of us to avoid stairs, or to sit up front, which if you can get would help, because the one thing it did was to allow us to show up 10 minutes before a show, meaning we did not have to wait for shows.
But believe me, my daughter standing in Pirates dark, boring back deck for 45 minutes when the regular line was only 10 minutes was not of any use to a child with BP, and standing for an hour at the back end of Space mountain or Splash mountain was not of any help to us, at least the line was moving when we went into the regular line.
I am not saying not to try for a pass, there are things that may help, although for our daughter not really.
Lastly, if you son is old enough to be allowed time on his own, this is when it because pleasurable for us with our daughter. The rules were set down before hand, we are going on this, you may go on that but we will meet back right here in 30 minutes. But this is when they are 13 or so, at 16 we were able to let her go by herself for an hour or two, which was great.