I definitely agree with WhydThatHappen, approval from somebody "cool" helps a lot. If he has a chance to perform in front of anybody else, each performance he does will give him more confidence and help him get over his nerves. If you can get some other family or friends to come listen, it'll get him used to performing and, hopefully give him a taste of the thrill of performing. It would be great if his band could do a dress rehearsal show in front of some of their friends, then they'll have some groupies already before the big concert.
When I'm teaching circus, I help my students pick a theme for their routine, and then have them practice in character. Sometimes having a persona you can assume when you get on stage can really boost your confidence. It makes the whole performance an act, so you don't have to feel self-conscious or personally judged, instead you are just playing a character. If this is his first time performing, he probably shouldn't go too over the top, because you don't want to give him too much to think about, but if there is a guitarist he admires, it might help him to watch a video and practice pretending to be that guitarist sometimes. Depending on the type of performance, you (or his teacher) could suggest they pick costumes for the show, to get them in character and help them feel like "real" rock stars. My circus kids also get a big confidence boost when I video tape them. Once they start to sound and feel good, tape them doing a dress rehearsal, then let thm watch it so they can see for themselves how good they sound and how cool they look.