If legal is not the point, than why does the son owe the money to the dad.  If he does not legally owe the money, than he does not owe the money.

One could just as easily assume the dad needs to prove rightful ownership.  The fact that the son has the money, means it is his unless proven otherwise.

So you are really talking a moral issue.  And one could also ask the morality of the dad asking the son for the money when the mom left it to the son.