I don’t know how much scintographs show, but they might pinpoint which area bothers him the most. I would make sure the saddle isn’t bothering his back too, but it does sound like a stifle issue, by the dragging toes in back. Usually they advise working a horse with stifle issues by trotting up hills and walking down (the uphill is what helps). Going down puts stress on his stifles, so you need to go easy down hills. Muscle strengthening helps support the stifle joint. Clear it with your vet first. Also there is something made to ice the stifles, made from neoprene I think. I am not sure who makes them but check Schneiders Saddlery and Smartpak to see if they have it. I will look and let you know if I can find it. Limit tight circles and deep footing, also no heavy work or cantering until he is better. My horse “popped” her stifles and I gave her 2 years plus off and she got all better. Needed time! She was worth it! I drive her with a cart. She is very forward and pushes herself hard!