P.B.
Hi K. - I also am allergic to Penicillin, Amoxicillin and then had a reaction to Bactrim - same itcing, hives, difficulty breathing, etc... Doctor in the emergency room said I had an allergic reaction to Bactrim and should stay away from all sulfa based drugs. The reaction will be worse each time I am exposed to it. So, I don't know if this is a commen link but I think you should definately follow up and get a second opinion, and also take him off the bactrim in the meantime as this is nothing to mess around with.
Good Luck