Stifle issues?

I have a 5 y/o OTTB and I am not sure if he has stifle issues or what is going on with him. He always seems sore when I pick up his back right leg, and when he is ridden he "buckles" it. Feels like he is tripping but it just looks like his Pastern and fetlock are giving out, I think it might have something to do with his stifle though. He also has a lot of trouble with his back lead on flying lead changes. He knows when I ask what to do because he changes the front lead right away, but always has to trot a step to get his back. I can't afford fancy vet procedures, but I was looking for a cheaper alternative maybe if someone has some advice? This is my first post but I am going to try and upload a picture of him "tripping" with a little girl I had ride him. Any advice is much appreciated. Thanks!

Could be stifle, could be back, could be front foot issue. When there is no obvious sign of lameness it becomes really hard to tell. One thing you can do is take the cap of an ink pen, and (don't press hard - just push enough to have a light pressure) run it down the horses back next to the spine. From withers all the way back. Do it on both sides. If there is back pain, you should see a visible spasm or the horse should dip to get away from it.

If you find back pain don't automatically assume that is your issue because many times back pain comes from compensation of something else.... That's a place where I would suggest your start though.

It's hard to automatically assume that the issue is a stifle though from the symptoms you are giving because they can be caused by so many things.

I would have the vet look at him. Also, post a video on here so we can see.