This whole thing sounds bizarre. Something seems 'off' with this vet. I'm wondering if something happened during the surgery and he is aware of it and you are not. It makes no sense for a vet to suggest putting him to sleep without exploring other options as you have suggested. I would find a new vet as soon as possible, perhaps one that you haven't yet called, and have your records sent over to them and start over with a vet that isn't biased. I hope that something can be discovered so that you can get to the bottom of this mystery. You must be very scared. Hang in there.