I would assume he’s eating it if they’re nowhere to be found, and I’d think he could get sick from eating plastic since it can’t be digested. 

Does he have other things to chew?  Some bunnies are just chewers..I’m not sure of any way to stop the behavior, but they do have disposable litter boxes…using those would at least allow it to be safe for him if he does eat it..of course those would need to be changed out pretty often and that would get expensive  

The only thing I can think of to stop the behavior would be to give him enough other things to chew..maybe hang a chew toy that he likes right by the litter box?? Then he could chew while he does his business if that’s what he likes to do.

Also is he neutered? Neutering can calm the bunny down and limit the destructive behaviors.

you could try putting some Phooey on it, it is a scentless bitter spray to deter chewing. It might not work though, Fern was not deterred, but my dog was!

I would recommend getting your bun a cardboard box for his cage – those xerox boxes are a great size and I usually put the lid on them and cut a little hole just big enough for a bunny to get in and out. Then they usually go crazy chewing the top off and making the hole wider. I bet it would really distract him from eating the litterbox.

I would say rabbits probably stop growing after about a year? I adopted most of my bunnies when they were adults, so I’m not really sure, but I think that 1 year is probably about the right age.

out of curiosity – does he have hay to munch on while he’s in the box?

You could try wiping a small amount of white vinegar on the box edges (it’s plastic, right?) to give it a repellent smell. But then put in more litter and put hay on it so when he wants to chew, he can chew the hay in the litter box instead. Alternative chew toys like Moo suggested are good too.

I would try some of the other suggestions- but if you just can not get it to stop switch to a metal litter pan- either a cake pan or another type of metal pan from a restaurant supply store.