If he has a favourite thumb to suck, tell them this before they put a canula in and they will take care to use the other hand or arm.
As he is so tiny they will almost certainly use gas to anaesthetise him and then put the IV line in once he is asleep. It can be traumatic if they struggle against the mask, and babies struggle against everything, but the struggling actually makes them breathe more and go out quicker. It actually takes less than 5 seconds but feels like about 10 mins.
Sleeping on a children's ward is terrible, it has to be said. They will give you a camp bed to put next to DS's cot, but the noise, the lights, the constant monitoring - it is not relaxing. At King's they try and put parents with babies under the age of one in a private room, so you might be lucky, but it's still noisy etc. Do arrange for visitors to bring you fresh nutritious food if possible. There may be a parents' kitchen where you cna make tea and coffee, with a microwave.
You will need to take your own nappies, formula etc.
I do know what you mean about the horror of the visceral mechanics of the operation, and I think that is completely normal - after all every bit of his little body was so recently your own, and he is your baby and your whole instinct is to protect. I was near hysterical inside, at times, when we had to decide what to do wrt bone surgery.
You just have to keep forcing yourself to focus out and take the longer view of the bigger picture.
And remember that on paediatric wards up and down the country, up and down the developed world, tiny babies are having successful, event-free, operations that improve thier lives, and which they sail through. Right now thousands of small children are in surgery, and it's all part of the normal working lives of the skilled professionals in charge. They take huge care over our little ones, they really do.