Supplementing my horse?

Hi, I have a 14.2 pony who is on very limited grazing during the winter months due to the fields being boggy. I ride him 4-5 times a week and he has quite a lot of energy (sometimes too much!). He is looking a little on the skinny side and his coat is looking a bit dull. He gets fed Sugar Beet, Chaff, Bran, and sometimes oats. He also gets pony nuts and carrots in the day. Is there any supplement I can give just to make him that little bit healthier? I have heard that cod liver oil is good, but would he want to eat it?