You would have to take him to the vet, but it looks like either a bad burn or a skin infection/fungus. Is he able to get close to either of his lights?
He can get about 8" or so away. This is on his side. I'd think if it were a burn it would be near the top of him where he's closest to the light. I could be wrong. Either way I think I will probably move his basking branch down a little bit. If it is indeed a burn, can I do anything or should I just let it heal on it's own?
We can't diagnose this just with a picture. It could be a burn (chams tip themselves toward the source of heat so a burn could occur on their side) but I can't tell for sure. It could be fungal or bacterial. A vet will need to test for these in person. Many fungal infections will fluoresce a characteristic color under a medical blacklight. But others don't. A culture should determine what if any bacterial things are going on. Until you have a vet check it DON'T put anything on it. Bring a fresh fecal too to find out what parasites are also present.