If you can turn out on good grazing then that would be a great starting point. In addition it would be good to get some alfalfa into his ration as it is a natural buffer to acidity. There are some pelleted versions of alfalfa that you can use: pure Alfalfa Pellets can be fed dry or dampened with water if he prefers them that way or Alfa-Beet which is a combination of unmolassed sugar beet and alfalfa which must be fed soaked before feeding. Although a horse would tend to consume a pelleted/soaked version of alfalfa more quickly than chopped fibre and therefore spend less time chewing, the main aim in this situation is to increase fibre intake and find a form of fibre your horse likes. Once the ulcers have healed you may find your horse’s appetite picks up a bit and you can try introducing some chopped fibre again.
For horses and ponies with a history of laminitis, feeds such as Hi-Fi Molasses Free mixed with Alfalfa Pellets or high fibre cubes can be a lower sugar option than those based on grass. However, they won’t be as digestible and so a horse or pony may not do so well on them which may require some additional oil to be added to help maintain bodyweight safely.