It looks like he's just getting older and starting to shed in parts, instead of having a full body shed. The older they get the more they shed in patches and the longer between sheds. The adults can spend weeks just shedding one area. He looks healthy!
Do you know how hot it is under his heat lamp? Also, what sort of bulb are you using in the heat lamp? I think if it were a burn it would show on his casque too, but best to be sure.