Intact adult male cats have large amounts of testosterone flowing in their blood. The hormone has many effects on the body. It promotes increased muscle mass. It promotes activity. It is linked to aggression. And it creates a strong urge to mate.
Many intact male cats make challenging pets. They may be obsessed with getting outdoors to engage in trysts. They may be very territorial, and they may fight with other cats and develop abscesses or catch FIV/feine AIDS. They may spray urine to mark their territory. When this happens indoors, it is quite intolerable for the people living with the cat.
When male cats are neutered, testosterone levels in their bloodstream steadily decrease. In many instances this causes their behavior to change. One of the most consistent changes is a decreased desire to go outside to look for fights and mates. Many, but not all cats become less active and less aggressive after they are neutered. Spraying of urine usually, but not always, diminishes.
Based on your description, I think your cat’s behavior is changing normally in response to decreased testosterone in his system. However, if you are worried about his condition, I’d recommend that you have your vet take a look at him.
The vet should be able to rule out complications such as infection that might be impacting your cat’s behavior (these sorts of complications are rare). And your vet will be able to help you devise a plan to ensure that you continue to enjoy your bond with your cat.
Hat tip to Mr. Gato — great photo!