Hmmm, i used to have a rabbit that did a similar thing, he used his litterbox when he was young and then wouldn't use it when he was full grown. Are there any strange new smells where the litter box is? Also, did whoever was looking after him make him use the litterbox?

I have to ask, does he pick the litterbox up in his teeth and throw it? My rabbit used to think it was SOO funny to do that! Sorry i've not been much help, but i'm not that experienced in the bunny department !