Should I continue to help him or is that no longer possible?

A very good friend of mine is very thin. He is 16, about 5'9" and weighs about 125lbs. He wasn't like that before and sometimes doesn't eat anything and then just drinks coke or something. I feel so sorry for him because he is just skin and bones with no muscle mass and used to be normal. Unfortunately he is even proud of the fact that he weighs so little. He has had fainting spells several times because of it. Once we were out and I had to force him to eat something because he was almost fainting and was completely exhausted. He almost wanted to hit me because he was angry but I managed to calm him down. Unfortunately he doesn't want to talk to me about it but it hurts to see him like this.