I'd contact the school and see what compromise you can come up with. I was unaware that there was a microwave available to the kids, all they had to do was ask, until a friend of mine started working for the school. Ask your son's input. Explain the situation and see if he has any ideas for his lunches, he may surprise you.

It's amazing what's out there in terms of lunch boxes etc. You can put left-overs that taste better heated in a thermo that has been previously warmed with boiling water and at lunch time it's still warm. Not hot, but warm.