Last February (wow it's been a year!) my DD (then 3) had to have 5 cavities filled so they opted to send us to an oral surgery center. We just talked to her about her "surgery" to fix her teeth while she was asleep. Unless a lot of people around him have had surgeries, he'll have no idea what this means. We just told her that they would give her some medicine that made her go to sleep. Then the day of the nurses did a great job explaining to her about how she would have to wear a mask that gives her air and to take deep breaths. They even brought a mask to her for "practice" that way she could get use to it and it wasn't scary. She was allowed to bring a stuffed animal for comfort and they gave her little toys/stickers before hand for being so braved. I was able to stay with her until they wheeled her into the room and surprisingly she didn't cry.
To warn you, he will freak out when he's coming out of it b/c it's a crazy experience for kids. We could hear DD screaming before they allowed us to go back to see her but she wasn't hurt or in pain, she was just confused. So don't let that freak you out. Even once we went back to her, she was still confused that she continued to scream. It took about 20 minutes for her to snap out of it. THIS IS VERY COMMON!
Good luck! It's going to be harder on you than him.