Help with horse gaining weight?

We have got a 15 year old thouroghbred cross and the thing will not gain weight at all. We have had his teeth floated, the vet checked for worms, anemia and sand. We have tried using weight builder, beet pulp, corn oil, moles and everything else and nothing is working at all. Our vet set up a feed ratio for him and I know it sounds like a lot but as of right now we feed him 5 pounds of cracked corn and 5 flakes of alfalfa haay in the morning and the same at night. We moved him to one of our small paddocks to limit excercise and keep rounds bales of gr hay available for him at all times in it and the hose is still showing ribs. He is about 16.3 hh and some have said that because of the breed he will stay thin. Can anyone help all of our other horses are fat and have a nice barrell and then he looks abused or something.