You could be speaking of my own son. Exactly the same. Exactly (except for the masturbation....IF he did that, that's fine with me, it's not something that concerned me).

My son withdrew and we couldn't get him to articulate what the problem was. We thought it was growing pains, him being difficult, his twisted girlfriend, his tortured artistic soul, etc., etc., etc....
I also have depression but couldn't make the connection because, well, I have depression and it leaves you incapable of lots of things. I am now improved, somewhat, and so can recognize it in other people.

Finally we got him to therapy and on anti-depressants. He has improved to the point where he can recognize and tell us when he's going downhill.

Just my own experience.