Neutering will eliminate one reason for male spraying (sexual drive), but cats-- both male and female-- can spray even if they're "fixed".
Cats spray as one way of marking territory. Cats also use other methods to mark things. When cats scratch, for instance, they leave behind a scent mark from scent glands in their feet. This kind of mark says, "I was here". It's like you or I signing a guest book or writing our name on a wall... kind of like cat graffiti. Cats also mark familiar objects and people in their teritory by rubbing their faces on it. This mark says, "This is okay-- it has my seal of approval." There are scent glands near the whiskers, and when a cat rubs its face on you, he's marking you with a "stamp" that says you're okay! This is an affectionate gesture when done to a person or another animal in the household. When a cat sprays, he or she is saying, "This is MY territory-- all others, GO AWAY!". Of course, they can also say, "I'm available for 'services'; call me!", but if the cat is neutered before maturity, those hormones are no longer there and the cat probably won't send this message. But neutered cats can still say "GO AWAY!" through urinary marking.
I have a female cat who was spayed very young, and she still sprays when she is outside. Yes, urine still comes out. It's not much, but it's just enough to get her message out. I've caught her in the act and have seen the wetness left behind. I've had other spayed females that have done this also.
Now, having said that-- is there anything new around your house that might be stressing your boy... new pets, new baby, other cats outside, new schedule, anything? Cats may spray when they are stressed.
Barring that, are you ABSOLUTELY sure he's spraying, and not just urinating in places where he shouldn't? There IS a difference. When a cat sprays, s/he typically targets a vertical surface like a wall or door jamb. The cat is usually standing, tail up. As the cat sprays, s/he usually quivers the tail and moves the back feet as if marching in place. Then the urine is sprayed onto the surface in a pattern similar to squirting a spritz of Windex onto a window. There is usually just a small amount. In cases of inappropriate urination, the cat targets a horizontal surface like the floor, your bed, the laundry basket... The amount may be greater than what is produced in spraying, but not always especially if the cat is urinating frequently. This behavior MIGHT signal a urinary infection, which can quickly become a very serious problem, so it's very important to note the difference.
If the vet rules out an infection, check the cat's environment. Is the litter box (or boxes) in a place where the cat can easily get to it, and where he feels safe using it? Cats feel vulnerable when using the box, and if he doesn't feel "safe" or like he has some privacy, he might avoid the box. How about the type of litter? Many cats are particular about that. Many cats despise scented litters, and many have distinct preferences about the texture. Some cats, for instance, prefer the finer textures of clumping litter, others like the coarser feel of traditional litter. Some cats are very fussy about the cleanliness of their facilities. What seems "clean enough" to us might be the equivalent of a nasty gas-station crapper to him. Or, if you're using a covered loo and he prefers open facilities-- he may protest that (or vice versa).
There are many things to consider here, but be sure of what he's doing and try these tips.
Good luck!