No, I do not believe that he is going to Hell, but I could not guarantee it. The sin of masturbation can, I believe, be either venial or mortal. Certainly, saying that "it's okay; everybody is doing it" (the latter may be close to being true) is heresy, plain and simple. That is a mortal sin.
While teenage boys (as well as adults) struggle with this does not change the fact that it is sinful. Objective moral values exist, so what is sinful is sinful, regardless if "everybody's doing it." Ultimately, what is, is.
One solution to this problem (other than marriage) is fasting, but I do not think that it is something that could be recommended to a 12-year old. I would say, "Look, it is immoral, so don't do it, but if you do it, keep it to yourself, but never neglect to mention it in confession."