Billy Bones, or "Captain," as he likes to be called, presents a pretty fearsome sight. A degenerate alcoholic with a taste for bullying, loud singing, and swearing, he's a very intimidating man, someone you wouldn't want to meet in broad daylight, let alone down a dark alley.
And yet Jim Hawkins, whose parents own the Admiral Benbow Inn, at which the old sea-dog is staying, isn't all that intimidated by the new guest. That's not to say that he isn't afraid of him; it's just that he's less afraid of him than anyone else who knows the Captain.
To some extent, Jim's relative lack of fear is due to the fact that he's much more scared of the one-legged seafarer that the Captain's so often warned him about. Thanks to the Captain, Jim's youthful imagination starts working overtime, conjuring up terrifying images of this "monstrous kind of a creature" with just the one leg. To hear the Captain tell it, one could be forgiven for thinking that the one-legged seafarer is somewhat less than human.
Jim's attitude towards the Captain foreshadows how he'll conduct himself later on in the story. Far from being a frightened little child, Jim will prove himself to be a brave, fearless young man, willing and able to take the kind of risks that most adults wouldn't dream of taking in a million years.