Don’t feel awful for making him eat “just” hay! Grass has the optimum nutritional profile for buns, its skinny so they can eat a lot of it often, and that’s how buns (and horses) naturally eat. If you give him a good variety of grass hats (or mixes, most European hay is sold as “meadow hay” and that’s mixed grasses), he will want for nothing. You can of course give him a few of the new pellets -start slowly- perhaps handled as treats when you have cuddle time. Sometimes pellets are too rich for a poopy butt bun, so only give a few for starters.
If you can get him twigs from pesticide free apple trees and/or willow, that could add some enrichment to his daily life + some good fiber. Its rather like giving a dog a bone to gnaw on -it’s delicious, not fattening and a nice activity. Tree bark also contains astringent tannins that can help the gut. The first few times a bun gets a fresh twig the bun will only gnaw a little bit on it, but they soon become expert at stripping off the bark.
(CC is short for Oxbow Critical Care. It’s a recovery formula for small herbivores. I think Supreme Recovery Plus is more common in the U.K. Both are products for buns that won’t/can’t eat enough.)