Skinny thoroughbred! please help me!?

hi! my 13 yr old gelding is a pasture ornament. he shares nine acres with another gelding. fed high quality feed twice a day, with full choice hay, alfalfa at night, corn oil over feed, weight gainer over feed. four cans of feed a day. he is current on his fecal checks, dental floating. he is really skinny.. ive had him for six years and he has never been not skinny. im just tired of people coming up to me and asking me why i dont feed my horse.. the other horse with him is only fed a half a can a day and is very fat. he is a paint horse. thank you for looking