What are the best ways to get a horse back into shape? He is a 23 year old OTTB who used to do 3ft+ jumping but has been left in the field for a while. He needs help with his cantering and keeping his correct lead without getting tired and swapping. He also needs help knowing where to take off from jumps. When we trot in he usually takes off at the right place, but when we canter in he takes off really really far away, like an entire stride too early. This is why we can only practice with single jumps, no lines, because he always canters between the lines. I've been using small jumps, like tiny cross rails or 18inch verticals.
What other ways besides simply riding can I help him out? I'd like to show him but I'm not sure how to begin.