I agree with Lut and Casper and Kitty. If you spray him with water - you have to do it right when you are catching him in the act. Otherwise, he doesn't understand why he is being sprayed or punished.

Are there other cats outdoors that he can see? This might be a territorial thing - and he is spraying (marking) his territory.

Having him neutered will help - and the sooner you do it the better.

Good luck, I feel for you, as I have had the same problems over the years.