busymummy, my DS has had 4 GAs so far, and will have another 2 (at least) in the next year.
IME children sail through. The staff and anaesthetist are generally very experienced and it all happens v smoothly. It is VERY hard on the parent. They will ask you to cuddle your ds as he goes unconscious, and that is very emotional - but you have to keep calm fro the child - and it is awful leaving them, unconscious.
But for teeth it will be an incredibley quick procedure, and he will come round very fast. he may be woozy, and he might feel sick, although mostly anaesthetics these days aren't like the ones from the 'old days' and I think it's much rarer to be sick, now.
They will call you as he is beginning to come round - he won't ever be taken away from you while he is conscious.
Just be clear that you ask them about pain relief for afterwards - what they have given him, and what you can give. Remember you can alternate Ibriprufen and paracetamol.
Also, he will be hungry from not having eaten beforehand - so take something that he likes that he can easily eat - yogurt. And drinks - he will be thirsty.