We also have an HYPP N/H gelding and for quite a while it was hard to keep on weight. We now have him on teff hay (grass hay gave him runs and alfalfa gave episodes) and Triple Crown Light/Low Starch pellets. It isn’t cheap but it keeps him at a decent weight and free from episodes. It did take some trial and error to get the right mix for him as he is ridden almost daily so he burns up the feed he gets.
I agree with Joe-Joe, for us the diet has been critical for maintaining his health episode-wise. Except for a reaction to sedation recently, we haven’t had issues in well over 3 years.