My rabbit is litter trained, but he poops everywhere when he's not in his cage!?

He uses the litter box in his cage very well, there's barely any other poop aside from in there (just by his food where he stands to eat there are a few pebbles). But, when he's outside of his cage in his little pen (I set it up usually in the living room), it looks like a mine field. I've tried putting the litter box in there but he gets so excited he just bats at it and knocks it over. What else can I do?