Have you tried wiping him off with unscented baby wipes? I wiped my baby bunnies this morning when they tinkled in their box. Seemed to work OK. I've always wiped my chihuahuas when they got dirty or stinky and I didn't have time to bathe them right them.
But to answer you question, I don't think you are supposed to bathe bunnies ever. I believe it can put them in some kind of shock and I've also heard that some buns have died. I am definitely not an expert on rabbits, I'm just learning too, this is just what I have learned from my own reading.