What does your *vet* think... I mean, he can see the horse, we can't.
From what you describe though, if you have tried everything and he's still barely getting around and in obvious pain, it may be time to start thinking about euthanasia.
What are his living arrangements though? If he is currently stalled overnight (or more), you might try putting him on 24/7 turnout if you have (or can find) somewhere to do that with good footing (not a mudpit) and a good deep shed for him to shelter in when required.
Because, I gotta tell you that I was seriously starting to consider putting my older TB down about ten years ago because he was having increasing difficulty getting through the winters. By February he would be unable to do more than walk, really, and not having much fun with that either. But then we bought this place and he's been turned out ever since (tho I do bring them in maybe 2-3 nights a year, not so much for cold as for when it's a driving heavy wet snow from the south that gets into their shed and makes them wet and cold for several days in a row), and it has made a HUGE difference. And mind, I live almost an hour north of Toronto, so this is not just something for warm climates
He still gets real stiff on those nights when he *is* stalled overnight, or even if I just put him in for an hour to wait for the farrier; and he is still arthritic of course and when we have many months' worth of very hard ground I do start to think about euthanasia again (but more b/c of his feet than his joints)... but in general he is a different horse than back when I was boarding him, even though he is older now, and he trots and canters and rears with the other guys and does not seem to be in pain. I would not *ride* him, he is not *sound*-sound, but he is quite definitely pasture-sound and I totally credit that to living outside where he can constantly move around.
Best of luck,
Pat