I highly doubt your horse has Cushing's he's far too young, unless he's been seriously over vaccinated in his life. It can be caused by age or over-vaccination. In his case it's clearly not age.
"Equine Cushings disease is caused by a tumor in the pituitary gland, which is responsible for the production and regulation of hormones. Symptoms include a long, shaggy coat that does not shed, excessive drinking and urination, laminitis, a tendency for recurring infections in the hoof (foot abscesses), and a loss of muscle mass, especially along the topline and rump." Eye of the Storm Equine Rescue: equine cushings
I strongly suggest you read that link thoroughly, even if he doesn't have Cushing's if he does share some of the symptoms you could get the herbal blend (Bess's Choice) that website refers too, as it's an herbal blend not a medication it can't hurt! I have 2 Cushing's horses at our rescue and both are completely sound, mostly shed out their coats, and the one who had sight when we got them still does, the other is blind but has been for years before he arrived with us. We also give it to a few of our older horses who have trouble keeping their hormones in balance.
Good luck with your horse :)