He will reach sexual maturity between 6 and 7 months of age. Females sometimes reach maturity a little earlier at around 4 months but males do not. Therefore, it's important to get your kitten fixed when he is around 5 or 6 months of age before he reaches full maturity. There is some risk in doing it any earlier than that, although many shelters do neuter much younger for reasons of population control. But for a cat in a home, I always recommend waiting until 5-6 months.
Sterilization is the most important surgery a cat will have in its life. It is an accepted form of birth control, but just as importantly it has medical benefits for both males and females.
Intact males may develop prostate infections that can be very difficult to treat. Though few of these are malignant, the treatment is still castration. While male cats don't get prostate or testicular disease as frequently as dogs, the production of the male hormone testosterone by their testicles creates behavioural problems. Intact males want to fight, establish territory, and roam to find females. If they go outside at all, they are at high risk for developing abscesses from fights and for contracting serious viral infections, such as FIV (which is similar to HIV in humans) or FeLV (commonly called Feline Leukemia which is eventually fatal). They are also more likely to spray urine to mark territory.
Sterilization of both males and females is the answer to stopping unwanted pregnancies and curtailing these medical ailments and undesirable behaviours. Males are neutered. This procedure involves the removal of both testicles, which is the source of sperm and male hormones. Neutered males cannot get females pregnant and they do not do male things, such as wandering in search of females to breed or fighting with other males. Without testosterone, the prostate shrinks away and cannot cause health problems later on.
Hope this helps. Best of luck with your "new" boy. I hope you have years of fun with him. I have 3 neutered boys and they usually become more affectionate after they are neutered. My oldest male is 19.5 and going strong. I've had him since he was 10 weeks old and he was neutered right around 6 months. What a wonderful life we've had together so far and I hope we have some more time left too.