This infuriates me – The stupidity and lack of common sense in OUR public schools (they work for us) shows no limits. Separate issue on schools regulating what kids are eating instead of parents regulating it (how, when & why did parents acquiesce this?) is a discussion for another time and post.
I would track down the Food Service Manager at the school/district today and tell her what the issue is. Request that she direct her staff to allow special cases, and let her know a letter from the doctor will be forthcoming but until then, cafeteria staff need to back the $#@& off your son.