I am sorry OP, I don't know your history with this horse and what he was intented to be used for. You said they said it was ok to put him to work, but what kind?
If it were me, I would train him for as easy a job as possible to maximize the chances of keeping him sound. I would say he is pretty much gauranteed to develop arthritis at the site, but whether or not it affects soundness will obviously be the issue. I would also hold off on antinflammatory injections in that area unless he is actually hurting. They should help but will eventually stop working, so you do not want to start with that unless you absolutely have to.
The interesting phenomenon about bodies is that many of us, human and critter, have deformities, degeneration and injury that may show up with diagnostics, but not ever cause the individual any pain...they are just randomly discovered like this case.
I would put this boy to work carrying light riders doing things that minimize working off the hind end, which sucks, because we generally *want* horses working off their hineys...not this guy.
My horse injured her left back suspensory last October. As I have recently started rehab riding her again, I am becoming very aware of thing that stress the hind end. So,tight circles, cantering-especially departures, sudden stops and starts, collection, steep hills, rapid backing, hindquarter disengagement, and more intense lateral work.
If this guy could have a career as beginner lesson horse or a recreational trail horse, I would suspect he would stay sound for quite a while, perhaps his whole working life. If you cannot find a buyer willing to risk it, how about a long term lease where he will be worked lightly and loved? That would help take the day to day expenses off you.
If you put this horse to work as a dressage horse, jumper, eventer, endurance, anything serious, I think you are just asking for trouble. I doubt his body will hold up long term. The good news is that he is currently sound. The main question is how to keep him pain free and useful as long as possible. Better a 15 year career as a trail horse than a 5 year career in eventing IMO.
Good luck my dear. So very sorry you are dealing with this!!