It is becoming warmer at the moment, perhaps he's just malting. My rabbit used to go pretty much bald on her back end because of how fast she lost her winter/summer fur.

If its not malting then another cause could be a lack of something in his diet. If your sure its not malting then I would try adding vitamins to his drink/food and see how you go, alternatively a trip to the vets may be better.

Hope that helps