It is best not to bath rabbits, at least not in the same way you would a dog. It's really bad for their fur and can, in mostly very untamed rabbits, cause shock. You should find the source of the staining and eliminate that. Is his cage dirty? Is he spraying, if so is he close to a female? Is he peeing on himself, if so why? Is he having trouble cleaning himself, if so why?
If you are able to/after you eradicate the source of the problem then they should go away on their own. If you want them gone sooner then that, you can mix baking soda and water into a thick pancake batter consistency and put it on the fur, let it dry completely and the brush out well with a fine comb. This may take a few applications and a very slight stain may remain depending how bad they are, but it has worked for me in the past.