Thank you for the feedback everyone.
Agree that it certainly looks like stifle joint. It does not appear to be locking but may have been injured and is slipping. If you hold your hand over the stifle joint (gently so you have enough pressure to feel if there is a grinding sandy feeling) and walk him off you can actually feel the muscle slip and kinda grind on the joint when they have an injury.
Xray is the only way to truly decide if athroscopy surgery is needed and I agree that if you are sure it is a recurring injury that would be the best course of action.
I will have to have my mom try to do that (feel his stifle joint) and see if she feels anything. I won't be home for another 3 months, so I can't.
I would most certainly be willing to spend money on an xray. The vets did mention wanting to possibly do that, but they did not have the equipment to do that at their clinic. But after seeing him, they were certain it was a stifle issue. They said they could actually hear it "popping". I've tried to listen, but I haven't heard anything.
He HAS been resting.
I would be hesitant however to exercise him to build muscle. It is important to give him time to heal and then to slowly build muscle and get him back to fit. We just went through a soft tissue injury of a stifle and we laid up the horse for almost 3 months before we hand walked over rails etc to encourage flexion and then over an added 3 months moved on to jogging. At the end of that time we were gentle loping and she is solid and 100 percent sound after almost a year.
For almost two years
!!!!! I only rode him about every 2 to 3 weekends for about 4 months this summer. Last summer, I rode him for a little over a month routinely, but we were not having this problem last summer. It only emerged this summer, and has gotten progressively worse (to where he is compensating for it by swinging his leg outward) mostly around November. He'll kind of have good days and bad days, as my mom reports to me, but in general, it is getting worse with pasture rest.
Did the vets suggest an inter articular injection as a first course rather than surgery? Of course they would have to decide the grade of injury etc. but for injury that has recurred generally the second time around quite often an injection will work well to tighten the stifle if it is an injury that only involves muscle slipping over a worn pattella. You might ask about that.
The vets actually did not mention injections at all. They only said to do hill exercises, and that surgery was an option but they did not recommend it because it is not 100% sure to fix it. I found out about injections by doing research myself. But it is on my "to ask" list.