You could move  him onto one of the Dengie Alfa A blends which you can use as a hay replacer, there are a few to choose from and I think the Alfa A Oil one is good for veterans.  You could also use high fibre cubes soaked into a mash and use that.  Try him with Allen and Page Calm and Condition to help build him up, if the hind gut is the problem then maybe some pre or pro biotics might help.