Juvenile hair loss

My son is going through hair loss, he is 13 and started losing hair about 3 years ago. It is not noticible and started with very little loss that only a mother could tell. I bought him a over the counter product and he has been applying but not much help. I was hoping this was just a temporary thing. 6 months ago I took him to the dermatologist and she gave us some special shampoo but I don't see a difference. His hair is still falling off, it is not noticible yet but I am afraid it will continue.
My husbands side of the family all have full set of hair. Mine to except that some of us have excessive "grease" on the scalp. It is called "seborrea" in spanish. I told the dermatologist about it.
Is this something from puberty maybe? someone else experiencing something similar?
my son is athletic and seems very healthy.
I am getting worried, please advice.