I remember when my son had his surgery for adenoids, they first gave him some nasty tasting drug that starts with a 'P' to calm him in preparation for when they wheeled him away to the operating room and give him the real anesthesia. They had him pick out a stuffed animal (a frog he named Ron from a book we had just read him) and then the drug started to kick in. It was pretty funny so see the huge smile on his little face caused by whatever that drug was.
The doctor told us that when they come out of the anesthesia following the operation that one of two possible things will occur. They will either be grooggy calm or they will be screaming scared and confused. Our boy was the later and it made me and W really apprehensive. You might want to prep your son a bit so that he understands that when he wakes that he wont know where he is but that everything is ok.
BTW... Ron the stuffed frog made a tour of Vietnam and Thailand that year. Even the Thai tour guides got a kick out of a frog named Ron.