stopped using his litter box

Hello, I have a 5.5 yr old lop ear bunny. He is potty trained. About half a year ago he started to pee around his litter box, and some areas in the kitchen. As a result, at night time I started to put him in his cage. Even in his cage he is not exclusively using his litter box, and peeing around it or some other areas. His bottom and feet are regularly wet, even though I give him bath, he stinks, and not very clean looking. We love him, and would like him to roam around the house, but it is getting more and more challenging (I am getting tired of cleaning up his mess). Any suggestion, how I can make him use his litter box again?

It is not usual for a rabbit to suddenly stop using a litter box. Sometimes this is indicative of a UTI. Could that be a possibility?
Hopefully you are only washing his feet. It is not advised to bathe a rabbit.

Any changes: new people, pets, furniture, new smells and sounds, new area to him, strange sounds outside, other animals roaming the yard outside, can all spark territorial marking behavior. Also if he's not neutered and it's territorial insecurity causing marking behavior, neutering may help.

If it's not a territory thing, it could be a health issue like blue eyes mentioned. UTI, e. cuniculi, bladder sludge, are some possibilities, as are some older bun issues like arthritis. If a health issue is a possible cause and your bun has urinary incontinence, then he'll need to be seen by an experienced rabbit vet. If you are in a country that is coming into the warm season, fly strike is a concern for any rabbit with wet fur, so that will need to be watched out for until this issue is cleared up.
http://www.bio.miami.edu/hare/urinary.html
http://wildpro.twycrosszoo.org/S/00dis/Miscellaneous/UrinaryIncontinenceRabbit.htm
http://www.rabbitwelfare.co.uk/pdfs/Flystrikedontgetcaughtout.pdf (medical related photos)
https://rabbit.org/vet-listings/

thank you for the replies. I only wash his feet.
I will take him to the vet, to check for UTI.