Another thing to consider, if you have the option, is to keep the horse on 24/7 turnout, rather than in a stall. The horse will lose a lot of muscle just sitting in a stall. And, quick bursts across a pasture actually can provide different type of conditioning that you would typically provide. As long as he doesn't hurt himself
My hadn't ridden my mare but once before my horse's spring shots last year, and my vet commented on how fit she was. He was shocked when I told him it was 24/7 turnout that had fitted her up!