How long do I have to help my ex?

I've divorced my ex 5 years ago. Our marriage was great until he got hooked on pills after 5 surgeries. I waited for almost 2 years after divorce for him to snap back to a great guy that he was, especially, since we have a kid together but he just went a different route. He has a degree and can do loads of things from cooking to fixing pretty much anything. He's been working since he was 14 but quit his last job where he was for 15 years. Since then he's been unemployed. I don't ask for much just for him to get his life together. I don't even ask for child support. I helped with so much: money, food, clothes, dentist, car repairs, etc. All I keep hearing is that he has a job lined out but it never goes through. In the last two years he worked for 3 months. I don't think he takes pills anymore. Two years ago he was diagnosed with ADD and that doesn't help either. I know that he is a good man with a big heart, he really is but I can't support him for the rest of my life. I do feel sorry for him and when he cried today on a phone that his knee gave out and he is in pain it teared me apart to tell him not to call me anymore because I can't help him. What would it take for him to pull everything together? How can I just let go and stop helping him without feeling guilty? It's hard to say "no" when a person is down but so far all my help didn't get any results. I am sure there are others who were or are in my situation. Thank you for your replies.