The only reason I can think of that they'd want to keep him overnight prior to surgery is to make sure he has nothing to eat as the surgery must be on an empty tummy. All of our local vets release the males the late afternoon of their surgeries. I TRY to keep mine from jumping on sofas, etc. for a day or two but it never works. I've never had a problem with any of mine. Hope all went well since I'm a tad behind on responding to some of these posts (I'm new and trying to catch up so please forgive any foolish questions that have already been asked and answered!).