My son has pain behind his eyes when he moves them?

Eye strain: a lot of conditions can cause this pain,starting from eye strain,if he doesn't wear an eyeglasses it could be an undiagnosed astigmatism which cause his eye strain,if he wears an eye glasses visit a doctor to check the pressure of his eyes and if the eye glasses needs to be updated.

Orbital cellulitis ?: if this is orbital cellulitis, it is a serious infection that needs antibiotics immediately. If it is just eye strain, you need not worry. I would go to the pediatrician and have him examined, give them a good history of how long this has been going on, and you may also need to see an ophthalmologist, an eye doctor. Do this right away, to make sure there is nothing serious going on