beleive it or not some breeders are spaying etc at 8 weeks old and there are vets that do it. in some breeds of cat it can stunt the growth, but cross breeds it wont. i breed maine ***** and its best to leave it to the last possible point as they are one of the largest breeds. but having them done can make them soppy, more well behaved as their hormones arent all over the place and also stops spraying.
I had to have my young female cat done when she was just 6 months, as we had male cats sitting on our door step and I knew it wouln't be long before they came through the cat flap to get her.
The vet said, AFTER he had done it, that it was too young for her to be neutered!!! I don't know if it is the same for males though.
I personally would not neuter before 5 months. But I would if possible get them done before that if he started spraying. I have never had a problem with overweight neutered males. It is true that they are more loving when they have been neutered. You would not want to live in a house with male that had not been neutered anyway. They smell awful.
Neutering is the best thing for toms if you don't want to breed from them. The op's pretty straight forward, and although they look a bit sorry for themselves afterwards, they're soon racing around, back to normal. x