I think it's a bald cap. The cap bulges out a little on the sides where it covers his side hair and you can see a line where the cap ends and doesn't quite match his own skin color at the temples.

Yea, that would have been a little bit TOO impressive to have been a full cap. Except for the visible line where it is pretty clear that it is a partial.....

John, if you're out there, we're here for you buddy!