" I wonder if he is so easy to house train because he was raised outside and has never learned that peeing and pooping in house was even an option?"
I'm offering up more anecdotal evidence to your thoughts. My boy was also raised outside. Neither dh or I can recall any accidents in the house. My son just adopted a rescue pup. He was raised in a foster home where there were other dogs and a doggy dog to go in and out at will. My son does not have a doggy door but it took less than a week for the pup to train my son to let him out when he sat at the door.
Griff sound like a really fun pup.
"If you can't see his soul when you look in his eyes, then you need a seeing-eye dog"