I completly relate to how worried you are! I was a nervous wreck when Jimby went in to be neutered and we had only had him 2 weeks, I was already so attached to him!!
As Vicki said it is a pretty straight forward procedure when completed by a experienced bunny vet.
Make him a packed lunch of all his favorite things so he has lots of yummies to tempt him to eat again when he comes round.
Hopefully the receptionist will have told you, DO NOT FAST bunny tonight! He should eat right up until he goes under as bunnies are unable to vomit.
I would aso send him in with a blanket that he has at home and maybe his favorite toy as it will make him more relaxed if he has things that remind him of home.
Hope it goes well and he bounces back in a few days!