Is this still a Christmas story?
Assuming the story is set in the present era, a more plausible cover story for an unrecovered body would be a plane accident over the ocean.
And while the boy may be grieving and unhappy with his uncle, I don't think it qualifies as an adequate inciting incident that creates the objective goal of finding his father. All his uncle provides by taking over is to make a sad child more miserable -- more negativity. Which is good enough for inducing the kid to run away to escape his uncle.
But that's not the reason he's running away -- not the primary reason, anyway. In terms of his objective goal he's not running away to escape his uncle (negative); he's running away to find his father (positive). And he would only have that objective goal if he had reason to believe his father didn't die in the accident, that he may still be alive.
So it seems to me the inciting incident should be a discovery, a clue that gives him HOPE his father is still alive.