I am curious as to how you know how often your 16 yo is masturbating. Is he leaving 'used' tissues all over the house?
When I was 16, twice a day was the norm - morning and night. Once in awhile, i'd do it in the boys room at school.
Stop worrying - what would you do about it anyway?
In and of itself this does not invite concern, if it occurs in the context of normal development, mastering school and enjoying relationships with peers. Your son is at the high end of the normal spectrum re: frequency of masturbation - it's not as if there are strict rules or guidelines about this. The main issue to focus on is not frequency of the behavior, but the overall context in which it occurs.
I'm not sure I want to know how you know this. But good grief, doesn't this kid have something he can do to keep busy? Doesn't he stay busy enough with school, homework, sports and clubs? If not, maybe you need to get him involved more in healthy activities. It sounds like he's got too much time on his hands.