Ive had about 8 cats. All of them get neutered and spayed. He'll be a little groggy for any where from a few hours to around a day or so, depending. The vets should already give him a cone to stop him licking at the stitches. He wont need pain meds, the vets will have already given him some ^_^ As for when you get him home, just do everything as normal. No need to worry. Just set him up where he usually sleeps and bob him on his bed until he comes round properly. Dont let him outside untill the stitches have healed. Literally just do everything as normal ^_^ so food and water as normal, but just hold back from keeping him running around or jumping about too much.
It's a perfectly normal things and they heal up really fast. Just don't let him out in a couple of days, and make sure he doesn't lick the "wound" - or whatever to call it - too much.
I've never heard of ANYONE giving pain meds to their cats after being neutered, so I don't think that would be a problem. Just relax and make sure that if something looks strange (stranger then when you picked him up from the vets) contact the vets again.