I agree with Roberta about getting him a large cat litter pan to use, but I diverge in my opinion of what you use in it. First, remove the litter and hay from the cage bottom and leave the plastic clean and clear of bedding.
You want to be buying an odor absorbing litter rather than hay or newspaper that soaks up pee but doesn’t control odor. I would recommend Aspen pellets (Aspen Supreme) or Aspen shavings for the best odor control.
ONLY aspen shavings are safe – Never use pine shavings. That said, untreated pine pellets make an excellent litter, and a very cheap one is called Feline Pine and can be found in grocery stores for cats.
If you bunny prefers the softer litter instead of pellets (and you will know if he scatters them and pees outside the box) then try Carefresh Original, the cheapest of the Carefresh soft litter line. You can get huge bags for less money.
Just avoid the generic Wood Shavings products for safety as well. Use a hay holder that you can hang over the litter pan, so bunny can sit in the pan and eat hay at the same time. They often use the litter pan that way.