1. Diet--is it balanced and adequate to his needs? What exactly do you feed and how much (by weight)? Is the hay good quality?
2. Deworming program--many people deworm but they aren't doing so effectively due to the drugs they are using. What do you use and how often? What kind of situation is he in--stalled, paddock, pasture, dry lot? Do you pick up feces every 2-3 days? What are the extremes of weather you see in your area?
3. All of the symptoms that you describe could be attributed to metabolic diseae--have you looked into Insulin Resistance? He's terribly young to consider Cushings.
4. Has your vet performed a full exam including neurological tests? (walking in a tight circle, backing, walking downhill with head raised, etc) Has your vet performed any bloodwork?
Licensed Veterinary Technician