If it's just a normal neuter you shouldn't have to keep them apart when he gets home. Since they are keeping him the night of the surgery by the time you get him back the next day he should be feeling much better. Rocky was 100% within 48 hours of his neuter.

Hang in there, I know how tough it is to have them away at the vet for surgery. I missed R&R so much and they were only gone a little over 24 hours.