I believe he is marking his territory or in this case your's!
It smells like you so he is in effect marking you as his territory or at least that is what I read it usually means on the house rabbit website. It is a kind of compliment like pooing on the bed too :?
I would probably consider getting him neutered if he isn't already as he is about the right age when the hormones kick in and then he may demonstrate spraying not just a neat puddle :lol: it is supposed to improve litter training and reduces territorial behaviour.
Caz