Yeah, I see from the pic the scars and the broken place in his hoof wall. I would guess he got cut by something; wire, trailer, etc. There is no way to know. If they leg does not appear to be causing him any pain, he could have some scar tissue built up along the tendons or muscles in the leg that restricts it's movement. Stretches should help some but scar tissue does not stretch and flex like other tissues so he may always have the choppy gait. If he is sound, then ride him. Lots of horses get bored if they are retired before they are ready to quit. Riding more often may help to keep the leg limber too. I don't really know what kind of competitions he could be in though, maybe horsemanship or trail classes? Good luck and keep us updated.