Currently discussing with my father in law who says he was a legend created by US politicians to push a political party and encourage 19th century American assimilation... I think that’s far too cynical, curious if there’s truth in that.
A quick search of this sub finds the following posts that may provide some answers.
Edited to Add: I realized that the link did not address your father in laws claims, which are outside my knowledge but here is another thread from u/itsallfolklore covering Coca-Cola and Santa Clause and Popular Culture which may be of interest.