It's completely, totally, absolutely normal. The masturbation (3 x day) is very typical for his age. It's much more unusual -- but also very, very sweet and impressive -- for him to breathe a word about it to you. I think it's very, very important for you to reassure him that this is completely normal and there's nothing wrong with it. It's the way teenage boys "blow off steam" before they're mature enough to have sexual relations.
Just say, "Sure, maybe you will [have a high sex drive one day]; it's probably too soon to tell. But what you're talking about is part of the normal changes in your body at your age. It's okay to do what you're doing and to feel the way you do." Say this calmly and cheerfully, like it's no big deal, which it's not.
Also, I usually refrain from disagreeing with other moms who post on a given thread, but the moms who are saying it's weird or abnormal for him to tell you about this, or that teen boys don't "have to" masturbate -- I'm sure they're nice people, but they are not well informed on the topic of teenage-boy development. Please, please do not make this poor sweet kid feel ashamed! Please.
P.S. Oh, when I was in jr. high, the health teacher strongly advocated a book called Changing Bodies, Changing Lives. I was too embarrassed to even think about said book at the time, but from what I recall of her talk about it, it might be a good book for your cousin. This teacher went on and on about a chapter on masturbation, which to me at the time was a mortifying word just to *hear,* but clearly there's a need for books like this! Try hunting down a copy for him.