Pimples on scalp

As I have said before, my 16 year old has problems with acne. A couple of months ago he started having a problem with pimples on scalp. Since we can't figure out why he gets pimples, I thought this one might be a mystery, but it wasn't. I figured out that it was caused by him wearing his hat all the time and his sweating, so at least this one was easy to fix. As for other acne, there are all kinds of things that it gets blamed on, and most of them have been proven false, I think it's stress and hormones myself, during exam time his face was a mess, I don't think that was a coincidence.