Worried about my son

My daughter goes through this from time to time with her two boys who are autistic. Used to be, like someone mentioned, chicken nuggets and cheese pizza now and then, and then there are times you can't get them to eat anything. The younger boy quit eating once because it hurt to go #2 and he figured if he didn't eat there would be no #2. The older one started getting a complex about his weight and wouldn't eat normally but used to sneak food, especially cheese...Sometimes the medication they give to these kids really has some side effects that cause some of these problems.
Good luck to you, I hope everything works out OK...