The first thing I would do is a get a better farrier/trimmer. Those feet look quite unbalanced. Hooves that aren't well balanced can and will make any other underlying issues worse.
I also agree that his injury as a young horse and likely unbalanced feet at that time as well, made his tendons/ligaments not develop properly.
As for treatment, I would want the leg ultrasounded to check for soft tissue damage and then go from there. Have the vet look at both feet as well, to check for hoof balance/function.
If the new vet can't find anything, I would put him on a good joint supplement like Cora-Flx and add an extra dose of 10,000 mg of MSM. Keep him booted during work and limit him to walk/trot. I would pasture him only with other "calm" horses to help reduce the running/playing. Maybe some older pissy mares
Good luck and keep us updated when you get him looked at again.