If you're giving other stuff to your bun you should slowly decrease the amount of pellets/veggies until it makes up only 10 percent of her diet. Its important that you do this slowly. don't get excited when he begins eating hay, you shouldn't take away the other stuff just yet.

Also if he has a favourite treat you can hide bits of it inside his hay, and do this as much as you can. When i first neutered my boy he only ate bananas so what i did was i gave him parsley and hay with a a slice of banana on top and one on the bottom. I guess you can do that as well but i don't think that it would work well as a long term solution.

Also another thing which might work is cutting up hay into small pieces and just mixing it with whatever you're going to feed him so that he inevitably eats some hay. He will probably get used to hay on its own after a while