Grass is fine instead of hay aslong as that is what he's used to. Introducing grass suddenly can upset their tummy. It's handy to have hay as well and it's easy to store and suppliment grass with. Which ever use use he needs to have constant 24/7 access to it as rabbits eat all the time. That's why hay can be handy as it's not always practical to have rabbits on grass all the time. You also need a large area as the eat through it and you'll need to move the run around.
Rabbits would usually eat other plants with grass so thats why you feed pellets to get the extra vitamins/minerals. Do you have different pellets brands available - he might prefer a different type.
Herbs are better than fruit eg parsley, basil, sage etc.
Tam