My guess is that it's the structure of his head causing the breathing noise. I've known people with bulldogs and boston terriers whose dogs were like that.
If it's structure, you will most likely have to be very careful to keep him from overheating either through activity or external temperature...shorter noses don't allow for as much cooling as a longer snout. There might be other concerns, too....I'd read up on bulldog care just to be sure. Things may not affect your dog as much but it's better to be prepared.
I'd think your vet was right (assuming you have faith in her) since she examined him that it was just the way he is.
I'd love to see a picture of him. I'm trying to imagine lab with a bully type face and all I'm coming up with is "cute". What's his name?