Don't worry though, happens to the best of us. When I first got Tubby his first mom thought he was a girl and named him Bitsy. I didn't find out differently until I took him in to be spayed. Vet told me "It's going to be kinda hard to spay him!" I just took my friend's word for it that Bitsy was a girl and never bothered to check for myself. Don't know how he went from Bitsy to Tubby though - must have been all those treats over the years.

Either way I'm glad he's got a home, either with you or with the vet.