My rabbit is started to chew things in his cage he never done this before ..

he is a house rabbit gets out all the time but doesnt chew things when out he just sit at the fire like an old hound ....
So is there anything i can get him to chew on?? will it be his teeth he is 5 months old white mixed rabbit, have noticed its his corner litter tray that getting it...
Thank advise would be so great....
