Humperdink is getting better about using the litter box, but he will pee in another corner, or in his water bowl sometimes. We tried moving the litter box to the other corner, but then he just peed in a different corner. I've been soaking up the pee with a paper towel and putting that in the litter box so he gets the idea, but I am curious to see others' responses here.
Try spraying the area you don't want him to pee with water and lemon juice. Or come sort of citric acid. I have read rabbits hate citris and have used it to keep him from messing up the corners of carpet and from peeing/mounting my fiance's coat with success.
Downside is he may not use that area of the cage anymore, period.