Angie -- I know it is always frightening to put them under for any reason. But most vets are very good with this and neutering is a surgery they do a lot of. I just had Ben done a couple months ago and he is 4 years old. He was 4 pounds and did just fine with the anesthesia. He did have a day of feeling pretty bad, and another of still being very slow to move, but by the 3rd day he was running as normal. I had his teeth done too and they had to pull some so that may have been as uncomfortable for him as the neutering was. He really did come through smoother than I imagined. The stitches were perfect, never even turned red. Came out easy as can be. I could have done it myself but I did take him back in just so they could check him over.
Think of it as adding years to his life as you are saving him from testicular cancer and also reducing his chances of getting prostrate cancer. So, you are doing it for him.
As for size -- being over 8 pounds should give you some comfort, that is pretty big for a yorkie. I am sure he will do just fine. I do recommend the pre-op blood work. I got the detailed panel that they had to send off for Ben. It helps them catch existing conditions that may be a problem with sedation. That too will give you some more reason to be at ease. I also said yes to any question that I thought might make him more comfortable, like take home pain meds.
Make sure you are confident in your vet, that he has a lot of expereince with the surgery and has not had a lot of complaints. You can check BBB, but I think the best is to just ask around. If it is a vet you have been using all along, your own opinion is probably as good as anyone. I won't tell you not to worry as all good mommas worry when a surgery has to be done.
But I will tell you to try to relax, put him in God's and your Vet's hands and I am sure the little guy will be fine. It really is a pretty routine surgery. Let us know when the surgery is and we can join you in prayer and support.