Is he longhaired or shorthaired (he's the one you didn't post a pic of I think?!)? If he's longhaired then you could trim the fur around his bum area so that it's not as hard for him to keep clean.
My cats are shorthaired but if they ever have tummy troubles and have dirty bums (or feet!) I used unscented babywipes to clean up. They don't particularly like it but it's not as full on as a bath
I find the Huggies 'pure' ones are excellent, just the right thickness and moistness - they are also amazing at getting marks out of carpets, cleaning up accidents/spills, cleaning paintwork - I have a pack in every room