How much will he change? - Not much, although his urine won't smell nearly as strong a month or two after the operation, and he won't be trying to get out to mate.
How much bigger? - Hard to say without knowing if he has any particular breed, but generally speaking smaller refined cats (delicate looking long slender legs, body, and face) mature younger than larger cobby cats (legs shorter in proportion to the body and big boned, larger body size, round face), so while a 'refined' body type cat may be done growing by a year old, a larger 'cobby' cat may not reach full maturity until 2 years old or more. That's just a general rule though and does not always apply!
When can you hold him? - Check with your vet, but it should be OK to hold him when you bring him home, just be aware that he will be feeling a bit sore round his back end at first. When we brought Sonic home after his neuter he jumped out of the carrier and you wouldn't even know he had surgery that day, apart from looking a bit sore - his behaviour was completely as if nothing had happened, and he only looked sore for a day or two.
Good luck for the surgery, I am sure he will be fine and right as rain in no time, male cats usually recover very quickly from neutering as it's a minor procedure with a very small incision.