What's making you apprehensive about his reaction? Is HE apprehensive? Just be very calm and factual with him about the GA, and remind him that he has had loads when he was younger.
DS has had numerous GAs, the last when he was 8, and he was fine.
The staff will be very helpful and kind. Tell him they will put the cream on his hand so that he doesn't feel more than a slight sting or push when the canula goes in - and it will be far less painful than any football injury he gets when tackling! Bt do say it ight sting a tiny bit. They will let you stay with him while he goes to sleep, and it will be very very quick. They will also call you when he is coming round - DS doesn't alwayys realise that the operation has been done, so tell him it's all done with as he is coming to.
The anaesthetist will come round and talk to you before he goes in - ask him what pain relief they will put in place for when he wakes up. They can make sure he wakes up in no pain. And start a rehime of alternating nurofen and paracetamol asap, if they say that is OK. But I don't know anything at all about circumcision.
They tend to 'do' children in order of age, youngets first, so when you get there, you may be ther for ages before he goes in. Take a bag of surprise distractions to produce when he starts on 'I'm bored', especially because he will be hungry and thirsty. A comic, book, borrow a DS gae from someone that he hasn't played before, or whatever. Even a basic model kit - be prepared to work hard to keep him occupied.
You've done it before - stay strong, calm and focussed and you'll be fine.