you gotta catch him in the act with a squirt gun. Need only use plain water. I've used that spray you mention but the cat will learn more effectively if you catch him "going" as he's going...with a simple few squirts to get him wet. After a couple times, he will be cured.
One thing I don't understand though, how come you don't have a litter box set up so he can go in an appropriate place?
Get your cat fixed, if he/she is not.