How do you keep him on a good path? You can't. The only person you can control is yourself.
How do you be there for him? First, accept that he may never change and may continue to spiral down. If you've accepted that and think you're in a good enough place to tolerate being around him, then do just that, be around him sometimes. It's paramount you have good boundaries though. He may try to use you for money, shelter, or any number of things you will have to say no to, and you need to feel alright with saying no.