Is he bonded? Sealy was terrible with hay until he was bonded with Bandit, who was a guzzler. Ciara also wasn't fantastic, but she was fed on a diet of nothing but muesli by her previous owners and the vet told me her jaw muscles would probably never work properly as a result.
Otherwise, rabbits tend to like nice fresh stuff (if it has been sat on, peed on, breathed on for a period of time it becomes "spoiled") so giving smaller handfuls more often may prove more effective than large amounts more occasionally. You could also try going for the full on natural stuff- if you've got grass growing near you, you could go on a little ramble and pick an array of wild plants for a free bunny lunch!