Feed him bulk, like chaff/chop with a weight maintenance feed, like Allen & Page Veteran Vitality.

Part of the Barley & Molasses Free Range, this is high in fibre but low in starch making it ‘kinder’ on the digestive system. It also helps to maintain a healthy digestive system. It is highly palatable to even fussy feeders and has a good calorie level to help weight maintenance, as well as being fully balanced with vitamins and minerals. Feed him this about three times a day or twice a day.

You could also feed him a supplement like apple cider vinegar - available from many food merchants like NAF. Accompany feed with portions of hay/ haylage in between meals - you may like to soak this so it is easier for him to digest.