Adding to my thin tb's feed?

i have a 12 year old tb, the vet has been out and he is very healthly no worms/ teeth problems. he gets 3lbs of triple crown senior twice a day. 3-4 flakes of hay a day, he is in light/ med work i ride 2-4 times a week and is still ribby with a body score of 4. i was wondering if adding anything would help ie rice bran, oil, beet pulp would help gain weight without being hyper?