He's starting his sheds by rubbing that part of his head on something rough. Yeah, low humidity makes it a little worse, but sometimes the humidity's fine. If he does it shed after shed after shed, he'll end up with a light-colored scar area on top of his head. I have a pretty adult pastel that's given herself one of those marks.
It's really common---it's not attractive, but it's also not usually a big deal. When he goes into shed next time, you should try to keep the humidity up, and you should remove all rough objects from his enclosure.
You might offer him cardboard shipping tubes as hides during shed. He can rub all he likes on cardboard, and he won't hurt his head. And then you can tear it up and throw it away, and give him back his usual hides.
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