He may not stop his spraying even after he is neutered, especially if he has been sexually mature for some time. Also, some cats can begin to spray even if they are already neutered, or even if they are female, so neutering may or may not curb this behavior. But he needs to be neutered whether or not he sprays. His metabolism will slow down, too, so watch his weight, but there is no reason to assume he will get fat. Neutering does not generally alter their personality, except to possibly make them more affectionate.

Good luck to little Snow Cat!