I can't believe that he isn't allowed to have peanut butter at school because of others allergies. They are not the ones eating it! It is not going to touch their food! I would call the lunch ladies at the school and ask exactly why your son cannot bring food he will eat for lunch. I send peanut butter sandwiches with my daughter to school when she doesn't like the hot meal offered. They have never said anything to me about it, and actually they offer a peanut butter sandwich and milk to the students that do not have lunch money...the whole not being able to bring it sounds fishy to me.
For protein...does he like eggs or cheese? Yoplait light or any yogurt made with splenda has no sugar...I have seen the yogurt drinks with no sugar added at the Rainbow in West St. Paul. I am sure they have some in Minneapolis. One other trick one of my friends has to do (her daughter won't eat meat) is grind the meat up, in a blender and mix it with the rice-a-roni or just plain white rice they are having with dinner. She has no clue the meat is in there and loves it!