as boomerang says... if he is pissing on YOUR fence, then its wrong. you can take it up with him and threaten him with legal action.

you could be a right cunt and check out if anyone can see him whilst hes doing it, becuase that could be considered indecent exposure. especially if a child could see him from his bedroom window.

(i read a book in which an old man used to pee on his compost heap and the daughters of the main guy saw him and told their mom about it... he got in trouble)