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). 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. (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. m_turner says that this is a reason why female cats will 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, other tomcats, including fixed males, have a very strong parenting instinct, even stronger than that of females who aren't in litter. 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.
I and others have seen several toms who've displayed parental concern for kittens:
- The first was a cat my parents had named Will. My parents saw no need to get Will fixed; aside from this leading to the ruin of a wonderful cat, Will would bring female cats and their kittens home to be fed. He'd leave them in the bushes, then demand to be fed outside. He'd stand by while the mother and her kittens snuck out of the bushes to eat the food my mother left out.
- midnighter got a new kitten last year. They had three adult cats then: two fixed females and a fixed male named Oscar. The females accepted the new kitten grudgingly, but Oscar loved the little cat from the start. He'd groom her, play with her, and even let her mock-nurse at his nipples (not as bizarre as it sounds; I've seen other male cats allow this, too; fixed females won't go for it, apparently).
- Our little household got a new kitten a few months ago; he and our adult, fixed Tonkinese Simon have gotten on fabulously. They play together, sleep together, and Simon spends hours grooming the kitten. Meanwhile, our adult female cat, who had several litters of her own as a stray and took very good care of her children, does little but hiss at the kitten.
- m_turner says that his cat, Blacky, would teach new kittens how to hunt -- an education the mother cat typically gives.