Male cats tend to be friendlier and more playful than female cats; however, if you want to preserve your male kitten's sweet personality, you should get him neutered when he's about 6 months old.
Our family veterinarian said a good rule of thumb for kittens of unknown age is that they're ready to be neutered when their adult canine teeth start coming in; signs of testicular development is another clue that it's time to take your kitty in to get snipped. Young kittens have undescended testes, so a cat won't have obvious balls until he's about 5-6 months old. The lack of obvious external genitalia can make gender determination in kittens something of a challenge; generally, though, the undescended testes feel like a pair of peas under the skin if your kitten allows you to feel around on his backside.
Intact tomcats will start spraying urine to mark their territories when they reach sexual maturity (some neutered males will do this, too, if they're not fixed in time, or if they develop an urinary tract infection or irritation). Their "territory" will inevitably include items like your drapes and your furniture. Tomcat spray is truly foul.
Upon reaching sexual maturity, an unfixed tomcat will often become moody, aggressive, and unpredictable. He'll want to start roaming the neighborhood in search of female cats to impregnate, and he'll start getting into fights. Some outdoor cats (and they inevitably become outdoor cats once they start to spray) may disappear for days, even weeks. (Reader M. Turner reports that his cat, Blacky, disappeared for two months before they got him fixed. His walkabout took him 15 miles away from his home, and in the process he crossed two major highways.)
A cat that was gentle as a kitten may become truly vicious. Some intact toms retain their good nature, but most pet owners would do well not to take the risk unless they have a compelling reason to want to keep their tomcat as a breeding stud.
Some intact tomcats, if they come across a queen with a litter, will try to kill the kittens in order to get the female cat to go into heat again so he can impregnate her. This kind of murderous breeding tactic has been observed in other mammals, including mice, lions and apes. This is a reason why female cats will often share responsibility for a litter or two -- all sharing nursing and hunting -- so that someone is always home in case a hostile tom comes by.
However, some other tomcats, including fixed males, have a very strong parenting instinct, even stronger than that of females who aren't in litter.
Sometimes intact toms will bring females and their litters home to eat. If you have a household of adult cats and bring in a new kitten, chances are good that it will be one of the adult males who will "adopt" the kitten.
