Aww. Maybe you can introduce him to a new kind of sandwich. Also you could ask if sunflower seed butter or soy nut butter is acceptable since they're not tree nuts so likely won't have the severe allergic reaction a number of people have.

It's a pretty common allergen that can be deadly by skin contact in some anaphylactic patients (one of the common allergic reactions I've seen as an EMT and in hospital settings to food allergens). I work in the food service now and we have utensils specifically designated for foods that have any trace of peanuts. We have to completely wash down and sterilize any surface that came into contact with the peanuts to be absolutely certain that none of our non-nut products are safe for people that may have a peanut allergy.

In the cafeteria at a school there's no guarantee that kids won't be touching each other or their food so it's better to be safe. It's a small inconvenience but well worth it if it saves lives in the long run.