You could put a bit of water in his current feed to soften it up a bit. You don't need to change his diet, if you feel he is doing well on it, just improvise a bit, to help him out. I had a gelding who HATED eating 'dry' feed, so I would just put a bit of water in, mix it up well, and the grain was gone hence forth. He had no issues eating, just didn't like dry food...he liked his hay moistened too, but I could only do that when I was feeding him; when I moved him to the last place he was at, he had free access to round bales. He did just fine, just went to the water trough fairly often!
I do second the advice to perhaps get his teeth checked and floated if need be.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."