HI
Just want to let you know it's not your fault. People who are depressed sometimes cannot really be helped entirely because there's something faulty going on with them. I speak from experience, I went through a period of closing myself off from other people because of something that happened to me. It actually had nothing to do with others so I just wanted you to know that. Also what may happen, in my case, some people get paranoid and afraid of bothering others, they feel like a burden because they feel horrible about themselves, but it's not your fault.
Just try to be there for him, like when he reaches out to you, if he does, or be his friend in some way. You don't have to talk about anything, just suggest going out, if you are near him, try checking up on him consistently, even if he doesn't answer all the time. Start accepting that he might have a problem you cannot help him with entirely. If it get really bad to where his life is in danger or he is self-destructing, try meeting up with him and seeing what is going on in person. Check on him and make sure everything is ok, then maybe try getting him some professional help. In time, he might get better with enough support, medication, therapy etc. if required.
I hope this was helpful