Thank you for your thoughtful question, Sarah. Your first step should be to have a thorough vet exam to get the go-ahead to condition your horse. At this point in his life, I am assuming you would like him to be fit for health reasons as opposed to for competition. If that is the case, I suggest making the choice his by using positive reinforcement training and enrichment. How about teaching him to touch targets for a food reward? Teaching him to place his nose on a stable target (such as a traffic cone) can be a fun way for him to keep his body moving. If you use two traffic cones, you can teach him to touch one, then trot to the second and touch it to earn his treat.
I also love to use enrichment devices for horses. If your horse is pastured alone, you can divvy up his food into several small portions and place them in sand-free places around the pasture. Placing some food items up high and others in horse-safe barrels and balls offers other opportunities for exercise, stretching and movementall on his terms!
If riding is what you were thinking, I suggest some nice trail rides, on even ground if possible. A break from repetitive movement (like he would get in a small ring) will be helpful, and the changing environment of a trail ride will be enriching for him. Let him choose the pace.