If you haven't had him to the vet recently, he should be evaluated for any urinary problems which may be contributing to his spraying. A bladder or urinary tract infection causing discomfort, may be causing this behavior.
If he was neutered recently, he will still retain a small amount of testosterone in his system, and if he sprayed before he was neutered, this will take a while to slacken off. If he was not neutered recently, his blood should be tested for a hormone imbalance, which, if detected, will be treated to improve the situation.
I assume your 'pet park' is a cat enclosure outside, adjacent to the house. If he's outside, even though he can't roam, he may still smell other cats (strays, neighbors, etc.) and is marking territory. Try a cat appeasing pheromone called Feliway/Comfort Zone for Cats, which comes in both a spray and plug-in diffuser, available at major pet supply stores, and also online. The diffuser resembles a plug-in air freshener, but has no scent to humans. Feliway helps to relax stressed cats, curb territorial urine marking, and is useful in a multitude of situations. For more information, please see: www.catfaeries.com/feliway.html
If you say he sprays when he knows you're looking, he may be seeking attention. Try playing with him with interactive toys, like a laser pointer or feather teaser (fishing pole toy).
Clean all areas he's sprayed, with an enzymatic cleaner, to completely remove the odor, so he doesn't return to the same spots.
These products are available in pet supplies and online.
I hope things improve, soon!