Any reason for the circumcision? I.e. religious belief? Not judging just curious why you'd do it unless there was a medical need for it.
It's tough seeing kids get injuries or have to go through medical stuff. Our first had to have a lumbar puncture as a less than a month old baby and I was the only one that could soothe her as my wife wasn't able to go to the table, having to hold her whilst they do it was the single most traumatic thing I've ever experienced I think.
Since then she's managed to split her head open (whilst at nursery) and that whilst scary because of the blood and the size of the wound felt "manageable" for some reason.
Our youngest is a complete devil and has no fear the number of times he's escaped injury by being a complete psycho is reaching exponential numbers now!
We're very calm and not neurotic parents which has its pros and cons.
Ultimately you need to show a brave face for your kid and celebrate his braveness. Being honest as well is critical and just being calm and collected will really help. If you show too much fear and worry they'll vicariously pull on it and it won't help. They recover quickly and will forget about it more so than you. As parents we're lumbered with the worry and the memories of all the bad stuff whilst they get over things and move on very quickly especially at that young age.
It's not easy for some people, and there's nothing wrong with that, everybody has different takes on life and parenthood. Try and reassure him but be confident in it, perhaps celebrate post surgery with a gift or an experience or something you deem suitable. Whatever the reason for his operation (if you've had it as well say to him he'll be like daddy - sharing commonality really helps I find) you must display confidence and steadfastness, or at least try. Inside it sounds like you'll be running riot, which is understandable, but be calm and strong on the outside if you can.