Sorry I misread your comment to Tinyliny...LOL, "he's not exactly young", not he's young :) 20 is more than reasonable for arthritic changes and osteoarthritis...
Arthritic changes in any horse who has been under hard working conditions (ranch horses, showjumpers, dressage horses, reining horses, racing horses, etc..) are not uncommon as early as 8 years old. This is because horses are not designed to do what we do with them. They are supposed to spent 98% of their awake time walking and grazing and the other 2% running away from predators. Horses in the wild live to be about 20, maybe, because they are picked off by predators.
For our horses who are put to work at least an hour a day carrying things (which is hard on their skeletal structure as well), trotting and cantering for extended periods of time and who are expected to do this well into their late teens and early twenties, we need to give them some relief.
I would recommend the horse be outside 24/7 in a large field that encourages him to walk, a lot. Adequan injections are very beneficial, along with IV Legend to increase the quality of the intra articular fluid. If generic arthritis treatments aren't working to correct the problem, then you know something else is amiss.
For any horse, I recommend arthritis preventative treatment, if your pocket book allows for it. Starting any young horse on Adequan once a month can tack years onto their comfortable, useful life.