The best work out plan is one that both you and him can enjoy. Keeping him mentally stimulated is just as important as keeping him physically fit. Just because he likes to drive does not mean he cannot learn to jump or navigate obstacles too. Big horses enjoy trail riding, little horses enjoy off road walks or if you can, drive him (safely) out in pastures or on accessable public trails (if permitted). Lunging or round pen work as well as driving in the ring will serve the purpose of giving him exercise, but it can get boring so it is up to you to find out what else he likes and what you can do together. Every horse and every regimen is different. I would incorporate both practice drives to prepare for shows and fun workouts to keep him fresh and fit. You can adjust the duration and intensity of the workouts according to how often you get to work together, both of your moods, the weather and your show schedule. I try to give every horse in the barn ten minutes of exercise daily and twenty to thirty to the ones showing. Every horse is different, some of mine lunge, others ground drive, some jump and some just walk on the lead around the farm. Find things the two of you enjoy and develop a plan you can stick to.