I just joined this group today also. I posted about my 12 year old daughter and all her gluten battles including surgery on her intestines.
She had been cheating and caused her to have intussusception (bowel obstruction). Tell him if he cheats it is doing him more harm than he realizes inside his intestines. We can't see inflammatory damage.
My 14 yr old son should be eating better foods. He was allergic to wheat, eggs, and every other thing when he was younger and outgrew a lot of it. But now he is having intestinal problems - running to the bathroom with urgency after eating certain foods, primarily greasy foods. (even if made with olive oil) I just had a blood test for him with a celiac panel. Seems boys are even worse than girls with complying with the diet. It is a full time job but just keep educating him about the disease and the harm it can do if he cheats "even a little bit".
I agree with the other post - shut off the lunch account. That's one less worry. Not to say there won't be more (like if he goes to a friends house.