Do they cut the balls out if you neuter them and how did he act after?
Please note that they are correctly called "testicles"........balls is a locker room term.
Yes, the testicles are removed in a neuter. The scrotum (the sac in which the testicles are housed) will be empty and floppy for about two weeks after the neuter until it atrophies. After that happens, you will never know that they had testicles.
The testicles basically have two functions: they manufacture sperm and also testosterone. Testosterone is the hormone of desire and also aggression. Want to visualize testosterone causing aggression? Think about the behavior of a bodybuilder or weightlifter who is injecting lots of testosterone to build muscle and grow stronger. They often are not pleasant to be around, especially when they are combining anabolic steroids with the testosterone. O'kay, so the example doesn't exactly fit guinea pigs, but you get the idea.
Removing the source of testosterone in cavies calms them down and makes them less aggressive, especially toward other cavies. It doesn't seem to change their behavior toward humans. When I had Sparky neutered, I also had his cagemeate neutered so as not to upset the balance of power between them. It all worked out very well.