As far as the school issues go, get a doctors note with his diagnosis and ask the school to evaluate him for either an IEP or 504 plan. (And don't take no for an answer.)

http://kidshealth.org/parent/positiv...504-plans.html

It's a bit of a process with evaluations from various specialties, but it can be very helpful. Special services and accomodations can be provided with those plans in place. Also in the meantime, you can give the gym teacher a note from the doctor excusing him from any activities that he can't do. Hope this is helpful.