I have heard two things that could possibly work:
1. Neutering (which you've already done)
and 2. There is a type of kitty Pheromone plug in stuff that if you release it into the air it will calm the animal and make it less likely to spray. 
I have read other comments of making a pen for him, which is a good idea if your willing to do it.
Just remember, with any outdoor cat there is never a guarantee that a predator wont get at him. Even if hes behind a cage.
From the sounds of things outside life might be his only option.