I don’t think you should stay with him. Sounds like this particular fellow is always going to be going through a tough time. And times are tough for everyone right now, not just him. But that doesn’t make him your responsibility.
What you could do, before you leave is to do some research and find some affordable counseling that he could attend, or even his clergyperson or primary care physician. If you know him well enough to know what kind of money he has and how he spends it, you could draw up a budget and leave it for him. If you think he needs to move to a more affordable place, you could research those options for him.
Research (privately) all of the options that you think he could use, including things that you currently do for him that he will have to do for himself once you leave. Resources, people to talk to, budgeting assistance, lists of where things are, phone numbers, products that you buy that he may not know what they are from toothpaste to frozen pizza, coupons all things that he’ll need, etc. Every thing you can think of, then put it all together into a neatly organized binder, with tabs and everything. Then sit him down and let him know why you are leaving, and that you are leaving for good, but you care about him and you have made this notebook for him so that you are not just leaving him high and dry.
Then find your strength, whether it’s through therapy, your faith, your friends, your work or whatever, but leave and don’t go back. It’s probably best to cut all contact. If you can’t do that, let him check in with you, but DO NOT go back or lend money or go over there. Refer him to the notebook, or tell him you’ll get him more info if he needs it, but that’s all.
This might be one of the hardest things you’ll ever do, but you need to do it to protect yourself.