Hmm it may be as simple as he can smell where he has peed before so he is going there again. I know you would have washed the sheets and all, but is there any way the smell could still be lingering? Rabbits will go where they can smell that they have been before you see. He may have been marking by spraying before was neutered and now he can still smell it. If you see what I mean?
Maybe if you put a
litter pan in the bedroom and put some of his litter from his cage in it, he will smell himself in that and use that. Might take time and patience.
Good Luck
Catherine