The same dog was "in charge" ALL the time, in EVERY situation? I ask because generally, pack hierarchy is very fluid, which is why modern behaviourists doubt the dominance theory.
Because he could, probably. Dogs do what works. Older children and adults wouldn't allow him to push them over and steal etc, but smallest child didn't/couldn't do anything about it.
Vets, unless they have trained specifically and separately in the subject, don't generally have much of a clue about behaviour.
He knew what was expected of him when the rest of the family asked him to do/not to do something.
It's not so much the way they're spoken to - it's the likelihood that the command is going to be enforced/explained.
My dog does absolutely nothing my DP tells her to do. Not because she sees him as somehow inferior - but because he, being totally non-doggy, has no idea how to enforce a command. For instance, if he says "Sit", she doesn't. Because she knows that if she doesn't, nothing happens. He'll give her the bit of toast/whatever anyway. So, to her, when he says "Sit" what he means is "I shall now pause for 3 seconds, then roll my eyes, then give you some toast".