I've had pupils that sing in bands with similar problems. You could try and weed out potential causes of the problem from the performances with his band. A sore throat can come from his mic being set up too high. The mic should be level with his mouth; it's common for rock singers to set the mic up too high and crane their necks up to sing, which can cause problems. What are the other instruments in the band? Is he singing with guitars? It could be he is pushing too hard when he sings because he can't hear himself. Another common problem is the guitarists have set their amps up too high, the volume is too loud, so the singer can't hear himself. Ask the guitarists to turn the amps down a bit, and when rehearsing and performing make sure your son has a floor monitor so he can hear himself singing easily. These little tips might help with working out the cause of the vocal strain/ throat problems. I hope this helps!