Try and get as much off as you can - you can certainly bathe him but you will need to wear full body armour! It won't hurt him if he has to lick it off himself (their tongues and stomachs cope with licking their bums, after all) but you run the risk of him rubbing it off around the house. Use a dampened flannel so you can scrub a bit or old towel. If you can hold him down on the worktop by his shoulders with his bottom under your armpit, he can't do much about it. He won't like you for a couple of hours, but it'll be worth it.