I have three suggestions. 1) have the horse vet checked (especially his teeth), 2) have your hay tested, it may be low in protein, and 3) check with your local feed store, some have Equine Nutritionists, that will visit your farm, assess your horse and make a complete feed recommendation. You may just need to increase his feedings. Even though this boy is three, he could be going through a growth spurt and just needs more feed. I have a three year old gelding that is rather lean and lanky, cause he is still a growing boy.!! We have been breeding these horses for over 15 years, and convinced they do not mature until FIVE even if permanent papers are issued at three. Good Luck.