We know so little about this kid that we barely even know if it's a boy or a girl (boy). He doesn't even get a name. But we do get the sense that he has something important to do with the story: he's running a fever as a newborn the day the old man arrives, and is seemingly miraculously cured by the next morning.
The child grows up with the presence of the old man, so, for him, having a winged human in the house probably seems natural. In fact, he treats the old man like he would an old, patient, pet dog. They're kind of pals:
Before the child got his second teeth he'd gone inside the chicken coop to play, where the wires were falling apart. The angel was no less standoffish with him than with other mortals, but he tolerated the most ingenious infamies with the patience of a dog who had no illusions. They both came down with chicken pox at the same time. (11)
We can't learn much about the child from this description, but we can learn a few things: he's an ordinary kid who comes up with "ingenious infamies"—i.e., the equivalent of pulling the cat's tail or trying to ride the dog—and he has some special connection to the man.
Eventually, the kid starts school. He heads out into the world to learn to be part of society. And this just so happens to coincide with the guardian angel's dramatic exit from his life. Coincidence?
Maybe. We never get much information, and it's hard to know what the angel would even be protecting the kid from. His cray-cray mom? His crab-killing dad? Dementors?
If he is a guardian angel, then people probably should have treated him a lot better, instead of like an angel of death or a sideshow freak. And if it's a coincidence, well, they still probably could have been nicer to him, right?
One other question that remains unanswered: how is the child going to react to news that his pet has flown the coop?