As for cleaning him, you can try a warm (wet) washcloth or unscented, hypoallergenic baby wipes. If that's not enough, a butt-bath can be given [note: don't give a butt bath if it's true diarrhea, as it could be risky for a sick bunny]. A bunny should NEVER ever be given an actual bath except in extreme cases under the supervision of a very rabbit-savvy vet, as baths can cause them to go into shock and die.
For a butt-bath, fill a flat-bottomed bowl (or a sink, dishpan, whatever) with about an inch of warm-to-hot water (if you can't comfortably rest your hand in the water, it's too hot). A teeny bit of small-animal shampoo (not cat/dog shampoo) or Dawn dish soap (which is animal safe) can be added but is optional.
Hold the rabbit so that you're soaking just their rear end in the bath; keep them there for as long as they'll let you (hopefully 2-5 minutes) then once the poop is loosened, dip a washcloth in the water and use it to carefully work the poop out. Be gentle, as a rabbit's skin is extremely sensitive. Towel-dry the damp fur gently but thoroughly and make sure they're kept warm (75-80F is ideal) and away from any drafts until the fur - including the undercoat - is 100% dry.
If you can't get all of it out, you can potentially cut some of it out if it's not too close to the skin (stay a minimum of 1/3-1/2'' away from the base of the fur) - I strongly suggest using nail scissors or other very small scissors, as they're more precise and offer better visibility. If possible, hold the fur you intend to cut between the index and middle fingers of your off-hand to make sure you can't snip too close to the skin even if the bunny struggles suddenly.