You’re in a really sucky situation, through no fault of your own.
But the thing is–who would a prospective buyer be? You mention someone like that old gentleman–but it sounds like even though he just walks and grooms his horse, that horse is sound. People without a lot of horse knowledge or who are physically frail themselves aren’t always the best people to take on horses with physical issues, especially a young horse with what sounds like a possibly chronic condition.
So, a person without a lot of knowledge might take a chance on a cheap horse as a trail horse, but not know how to maintain the horse properly. Or a person without a lot of knowledge might go shopping with a trainer, and most trainers, even for a lower-level rider wouldn’t recommend buying a young horse with arthritis and a history of lameness. The concern is, even if he can stay sound in light work now, what about in a few years from now? Then he’s someone else’s problem.
I’m a runner, and I don’t think human physiology is a great parallel with horse physiology and sports in terms of technical trail running versus more manicured trails. Usually, it’s more of an instance of finding the right shoes and building up to the volume and stress of different terrain with a human. Horses seem much more fragile than we are, sadly.
In my area, people who ride definitely ride more often than three times a week as a rule, even people without their own horses who lease. I think for now, trying to get him more sound and maybe a care lease and seeing how he holds up is your best option. If you know a decent lesson barn nearby looking for walk-trot horses that might be an option (and I stress nearby, so you can monitor/visit and make sure he’s not used beyond his limitations).
I’m really sorry you’re going through this–he sounds like a very sweet horse.