If no arthritic complications (OCD etc) have been diagnosed via ultrasound/x-ray,
turnout and exercise are key. Correct, circles are tough on your guy, especially on the outside leg. Hills are good at a walk. If your horse can’t back up in a straight line at least 4 paces, something’s amiss internally on the short side. Ditto trouble going down hills. Check out hindquarter symmetry from directly behind-might reveal longstanding weakness on one side. Uneven trot on one diagonal indicates weakness/problem on that side. Work rising on the weak side more if you are hunt-seat.
I’ve used Adequan IM for 3 years on my DW mare & find that it has helped stifles. I’ve also had her get adipose stem-cell, PRP and arthriscopic surgery for OCD complications and she does pretty well now. I’ve ordered Pentosan IM & will substitute it for (unobtainable) Adequan as soon as it becomes available. Cosequin ASU seems to help too as does stretching her limbs forward and back (like in a flexion test.) Avoid steroids and NSAIDs-they hinder healing. Stifles are tough!