How much of a friend can I be?

I have a friend who is going through a ROUGH time in his life. Another friend called me up and asked if I talked to him lately. I've called him, left him messages, even left my phone number on his voicemail in case he needed to talk. I've heard he's been crashing and that scares me. I've offered my help, but no call backs. Whether he realizes it or not, there are people who care and depend on him being well. ARGH, do I just give up or wait til he comes around? I'm not stalking, not obsessed, just want to be a friend and make sure he doesn't go off the deep end...hope it's not too late!