I would try some alternative types of hay, some rabbits are fussy :roll: other ideas you could try are sprinkling his pellet ration into the hay, spraying the hay with a bit of diluted apple juice, mixing in a bit of grated carrot or readigrass or ordinary grass into his hay to try to encourage him to eat it. At his age (I think he's 3 months old? from your other post) he is allowed unlimited pellets, when they are older they should be decreased to an eggcupful per day, if they over indulge on pellets they don't eat so much hay.
If he hasn't had carrot/readigrass/grass etc before, introduce slowly a small amount. Carrot shouldn't be fed in much more than a tiny amount occasionally as it is very high in sugar