My son now 10 has been in and out of the hospital for severe complications due to ear infections. I can honestly say that I think all of your child's experiences will depend on several things....
What kind of kid he is? Is he frightened by new/different/loud/strangers? This may be the biggest factor when dealing with his experience. I think the doctors and nurses will do everything they can to manage his pain, but if he wakes up and is frightened by what he sees, his crying may be mistaken for pain.
Also...your reaction to this situation. He will feed off your anxiety, and if you are nervous he is going to pick up on that for sure. Of course, we are always nervous when our children hurt, and even more so when they have to go under anesthesia, but try your best to be as nonchalant as possible around your son.
Explain to him that when he wakes up he may feel like his head is full of stuffing, and that his throat might be sore. Explain that he may smell funny smells, or even that his belly may feel funny.
My son had to go under anesthesia a few times, and I found he usually complained about the smell (gas anesthesia still smells even after sx is over) I found that using the correct terms, and explaining what things were- helped him to feel just a little bit in control of things.
I have a veterinary backround, and therefore my kids and I are more at ease with medical situations, but I do think that your son will feel better if he knows as much as he can. My son was 5 the first time he had a major surgery and hospitalization. He's now 10, and has had 26 spinal taps with no sedation simply by being told what to expect.
I hope your son does well. You will both be in my thoughts.