Speaking as a professional who deals with these issues daily (which means I am hesitant to sweep broad brush strokes of advice since every person is different), I wish to offer some hope other than what some posters are offering. So a few thoughts other than be very careful (good advice):
If he is a veteran the VA has new and better programs at some VA's for substance abuse, homelessness, etc., that are more integrated and successful in their approach.
Involve the family, even if the answer is they don't want to be involved: you won't end up stepping on toes or re-inventing the wheel as to what doesn't work for your friend.
I personally have seen dozens of men and women who were given up on that suddenly change. Yes, even heroin addicts with 50+ detox programs under their belt and methamphetamine addicts with no teeth and brain damage. They generally don't become model citizen's, but may straighten up enough with treatment and luck to live in a group home or Oxford House, etc.
A steady income with a legal payee is vital -- so the checks are written for housing or necessities and not junk for their arm or nose. Looking into Medicaid eligibility, pensions, SSDI, etc. might be a way you could help -- but he needs a payee if he gets $$$.
I suggest at some point he will reach bottom (believe me that living under a bridge isn't necessarily the worst that can happen), and try to make sure he has a way to contact you when that day comes. My recommendation is that NOT be your home address or home phone number.
Hard as it is, try not to be condescending, judge mental or angry at him. If you want to help him, your history with this man (as opposed to your boyhood friend that is gone) begins at this point. Treat him with humanity tempered by some firm love and reasonable and achievable expectations. Everything begins with sobriety, but some assets like good friends, money, advocates etc. make sobriety more likely.
Protect your heart and assets and find help and advice where you can. The system is set to help those that want it AND have advocates for them. There just aren't enough resources to go around.
Best of luck...