Anyone here a cat psychologist?

Our dear, darling cat, also known as Charles the Destroyer, has develped an annoying habit. First I thought he was simply a wee bit incontinent, which given that he's five months short of two years of age seems rather unlikely, or absent-minded. He's taken to, pardon the expression, piss around the house.
There are a few places he's selected to do that: my kitchen chairs - good thing the old ones are gone and the new ones have detachable pillows. My mum's favourite blanket to cuddle up in while watching TV - good thing it's washable. Still, it's rather annoying, but manageable.
But yesterday evening, I caught him marking a doorframe.

Silly, stupid cat, you're not outside, you're in a place where people live, and people do no, repeat not, mark their territory, so you don't have to, either! Do that outside, by all means, but not in the house!

He's not allowed in my rooms any more. We really have to cure him of this somehow.