I wonder if the basking spot is too hot. From the pic, it looks pretty intense on the area directly under it. I'd recommend checking the surface temp there with an IR gun just to be on the safe side. Also, it looks like the back rocks are close enough that he could jump on the cage around the basking bulb. If he did, that might have done it if indeed this is a burn (I can't tell from the pics).
In the event this is a burn, rubbing some aloe gel (as in the sap from an aloe leaf, not an over-the-counter skin lotion) on the area probably wouldn't hurt. From other posts on this forum, it appears to be safe for helping to remove stuck sheds, and it's good on human skin for burns.