Our Persian kitten is around 4 months old and he does not seem to keep himself clean like our other kitten who is half Persian.
He constantly has poo on and around his ass as well as stuck in his fur, whilst the other kitten is very clean and tidy and is always grooming herself.
I assume he has the longer fur of the two, which can make toilet habits a little tricky! Is he a house cat - because if so, the lit tray'll need to be emptied very frequently so that you don't get a build up of faeces where he wants to go. Try using unperfumed baby wipes to clean him up at the rear end, and make sure that his diet's well-balanced so that his poo isn't too runny.
The messy one is male? The clean one is female? Dose the messy lad manage to hit the tray when he pees?
I was trying to make a purely sexist joke there - but it sort of fell flat.
My two eat Iams dried food and their poo is hard and not so messy. Merlin is a lady and has long hair and a clean bum. Maybe a change of diet will help.