Not to worry. I was on the fence about neutering my past boy pup and had to So it when he was 8 due to hormones making extra ligament laxity causing a likely anal Prolapse.
I got it done and you know what? He was the same boy, no changes. Never fat or lazy in fact the challenge was preventing him from jumping up for one week during healing. He had no pain after first day. Luckily he also never prolapsed unlike his brother.
PleasE remember girl pups can and do get pregnant as puppies and it’s terrible for them. They WILL breed if you do not get him fixed.
Visually, my boy was just fine too if your worried about it. Find a vet you trust and it will be just fine and you will have so much peace of mind.