Brother suffering from loneliness. Him!

My brother flunked in school after which he broke up with almost all his friends and became reclusive. He finally mustered courage to break out and pass school and then get admitted in college. But he kinda became anti-social. He didn't want to make friends and did some really strage things like 'stealing' lunch from classmates etc. But he was still under control, although he constantly kept complaining about feeling depressed, not good, lonely etc. He tried to search for his answers or relief in various self-help books and novels etc. I constantly kept encouraging him to make GOOD friends and talk to people. Recently I found out that he has started smoking. To make matters worse he made some 'bad' friends and has been doing night-outs with them every other day. We keep calling but he doesn't pick. He keeps coming back sometimes asking for help, making promises etc. but then again goes back to his old habits. We are really having tough time dealing with him and trying to help him