I kind of hate to say this, but maybe love and spoil him for a while and if he doesn't show improvement have him put down? That way he has a chance of improvement, but if he doesn't, well, at least he won't end up in a bad place. That would be my biggest fear for an unsound horse, or any horse really, to be neglected or end up in a slaughter house. ANY horse can end up in a bad situation, but a chronically lame horse has less chance of finding a good home.
You could also try to find that ideal home for him though....it might exist. I would expect to give him away (to the right people) just because they are taking on a horse with chronic issues and he will be high maintenance to keep sound.
So I don't know. It's a really tough choice. I don't envy you one bit. I have had older/unsound horses before and luckily they became that way one at a time and I was able to afford to keep a second horse for riding while keeping the other horse in light riding/retirement. I really hate to see people dispose of their older or unsound horses. It's such a sad, sad thing. That's why I feel like if it comes to that point, euthanasia might be better then sending them on down the line to gosh-knows-where.
It's a tough, tough call.