INTPs can go one of two ways when left to the dark side. We turn Emo: get overwhelmed by emotions that we cannot navigate, so instead we wallow like hippos in the ever-proverbial shit creek. This is an OK mode because eventually we get bored with all our sad sack music and feelings, just like everything else, and move on.
The dangerous mode is Full On "I Am a Rock": Utter isolation from feelings, people and our own internal being. You want an emotional desolation? Check us out in this mode. It stems from (at least for me) a driving desire to not be here anymore, either due to sheer pain or the mindnumbing, soul-grinding tedium of life. Probably the MBTI description would be disconnecting from our Ne and using our Ti as an escape rather than as a filter.
So what does that mean? In the first mode, if you're on good terms, a gently teasing person can make us realize how ridiculous we are. The second mode is harder, especially if they're far gone, but a good first step is always to turn his own analysis on himself: try to help him figure out why he sits on the net all the time. Why does he not feel happy? It's a tricky situation and it may honestly take a trained mental health professional to get him out of a rut, but you might be able to set him on a line of thought which leads to him concluding that he needs help.
We're an independent, bristly lot. Anyone who plays the "buck up little camper" game on us is going to come away quite bruised! We must make the move to pull ourselves up and the most anyone else can do is help us see the path.