I'm in school to be a mental health counselor...and I know that doesn't speak much to my personal experience, but there's a few things that might help.
Like the others said, schedule it as short a distance out as possible--you don't want to build up all the negative thoughts over a period of a week or so. Also, during that time, find things to do to stay busy, this is especially important on the day of the surgery. Take your dog in for the first morning appointment so hopefully you can get him back the same day. While he's there, do anything you can to stay busy...go to lunch with a friend, do your grocery shopping, clean the house, take your car for a wash, read, watch a movie, go to work...do whatever it is that is mentally absorbing for you. Also, when negative thoughts do pop into your head, think about all the pros of having him neutered--that you'll have a longer time to enjoy his life with you, and probably will have fewer health problems to deal with. So, a little stressing during his surgery now will save you frustration, anxiety, stress, and money in the future. Do your best to stay positive--if nothing else, call someone you trust and talk to them, it always helps to have someone there for you
So so sooo many dogs go through these procedures, and as long as you talk to your vet about their experience and expectations for your dog, there's no reason to think anything negative will come of it.