I've been wondering for quite sometime since my oldest boy Sherbet (who is now 1 yrs old

) ..always hv bits of faeces stuck to his anus. It gets me wondering after I compared it to my two other cats (Neko-chan & Salem), their behinds are always clean. Granted Sherbet is a Semi-Long Hair cat but the faeces doesn't stuck on his fur, but on the anus. Is this normal??
I wondered if I have to clean his behind everytime he goes to poop