Rabbits’ testicles can be withdrawn into the stomach-cavity “at will” f ex when they feel threatened, but the vet will of course check for that. If they deem him too young or a girl, they will of course just send you home =)
I was told to bring food + a little hay for my boys, rabbits musn’t fast and after the procedure, it’s important that they start to eat and poop asap, so it’s good if you get him some of his favourite food.
Back at home: If he won’t eat of his own, you may need to syringe-feed him with a mash of pellets + water or Oxbow CC. Give him water in an open bowl, as a bottle may be to bothersome for him if he’s in pain afterward. You may or may not get a perscription for pain-meds, some vets distribute a long-lasting pain-killer during surgery.
They can get cold after having been put under, so he may want a hot-water-bottle in his cage, wrapped in a towel, but see to it there’s room enough for him to move away from the heat if that’s what he wants.
Twice a day you should check the area for excess swelling, seepage, excess redness or foul smell. It’s easiest if you have sb else who can hold him for you. It’s helpful to take photos of the area, so as to have sth to compare by.
Best of luck to you!