Just copied this from the PDSA website for you:
Should I get my male cat neutered (castrated)?
Yes. Castrating your cat prevents him from fathering unwanted kittens when he’s away from your house. It also makes him less likely to fight, so reduces his chances of getting feline AIDS (FIV) (which is spread by bites).
Castration also makes your cat less likely to roam, which can reduce his chances of getting hit by a car, and he is less likely to spray urine in the house.
When is the best time to get my male cat neutered (castrated)?
The operation should usually be done at four months of age (but can safely be done when your cat is younger or older). Kittens can get pregnant from four months of age and many people do not realise that brothers and sisters will produce kittens if they live together unneutered. If kittens are neutered later than four months, some will have an unwanted litter – this is contributing to a cat population crisis in the UK.
Kittens have traditionally been neutered at six months of age, but veterinary guidance has changed to reflect latest evidence and neutering at four months is now recommended. Ask your vet when the best time is for your cat. For help finding a vet practice that neuters kittens at four months in your area visit this website
www.cats.org.uk.