people (read:women) who throw around the "gross" word often dont realize how mean it is. I hope you didnt tell your son you felt what he was doing/did was "gross."
being a slob about it is reprehensible (dirty tissues/etc on the floor or anything else in plain sight), but if you gave him any shit about starting to masturbate, shame on you. if you gave him shit about the results of a wet dream, shame on you. Now if he stood over his bed and blew his load all over the comforter, then your son is fucked up. Hopefully you didnt behave ignorantly and manage to make him feel bad about himself and thereby push him away.
even at 11 when I started masturbating I was very discrete, and I still felt embarrassed and guilty about it. Thank god I never had to discuss it with my mom. I guess they realized at some point I was masturbating because one day my dad bought me a book on puberty, sex, masturbating, anatomy, orgasms, everything. It was a good book and I stopped feeling guilty about being so horny so often. so, my advice? get him one of those books, but have the male figure be the one who gives it to him, and never bring it up. the male figure should emphasize discretion as well.