Tattoo ink generally comes off skin with mild detergent solution and ethanol or rubbing alcohol. You can try giving bunny a bath, but it stains, so fur might be tough, and the ethanol is going to make him cold and make his skin irritated if you overdo it. I might spot test a little Vaseline as oil can break down some inks--then detergent (shampoo) to get the grease off.
Maybe someone else has a magic idea, but I would be more than a little mad, because you may need to wait for it to wear off.
All that said, I have seen rabbits groom ink id marks off in a few days (of course when I didn't want them to!), so if he is friendly with anyone I would put him in a cage with other rabbits and let them go to town.