First, you need to take care of yourself. You seem to be worrying about this way, way more than is normal. Are you prone to anxiety attacks? If so, get yourself under control. Your son will just feed off of your anxiety.
The anesthesia is the most dangerous part of the equation. If he's been put under once before and was fine, this should alleviate the vast, vast majority of potential issues.
My 3yo daughter had tubes and adenoid removal in December. This is a slightly less complicated procedure and recovery, but is basically on the same level. She was fine going in, fine in surgery, had a rough period in the recovery room (nausea and pretty unhappy for an hour or so), and then was pretty much fine. She insisted on walking out of the hospital, insisted on riding the elevator before we left the hospital, was "out of it a bit" when we got home, and was pretty much normal the next day.
Keep the appointment and get it done. The short amount of discomfort certainly sounds like it will be overwhelmed by the long term health benefits.
Good luck.
ETA: And to followup to Zelda's response. I think they are far more liberal in giving pain medications to kiddos these days. Pain management has come a long, long way. If you keep up with the meds they provide to you, there should be some discomfort but little to no acute pain. (With us, they recommended an alternating schedule of tylenol/ibuprofen. They even gave us some stronger stuff for her, but she never needed it.) Just don't get behind on the meds. It's hard to catch up.