Antibiotics after surgery are a prevenative. Its to stop an infection before it starts. Its ok if he doesn't have it, just watch him for symptoms. And swelling, tenderness, or puss.
Pain meds can make him feel more comfortable. Some guinea pigs don't eat or drink when they are in pain. If he is eating and drinking fine, I'm sure its ok. I wouldn't handle him much because he's probably sore.
He can get the female pregnant right after she gives birth. If she's almost going to have the pups, I would separate them to make sure he doesn't back breed her. Also, after surgery for a couple of days you may want to keep him in a small area so he can't run around too crazy and rip out his stiches.
If your vet didn't say he needs to fast, that's ok. It depends on the procedure and what medications they use.
I'm glad he's not groggy anymore. Good luck!