I've found a record for Narcisse LaPorte born at about the right time:
Narcisse LaPorte born to Jean Baptiste LaPorte and Agathe Mace, on the 29th of November 1814.
Besides the date of birth here and the seeming absence of a birth about the same time of a similarly-named "Denis" child, there is the eventual settlement of Francis Nelson LaPorte in Beekmantown, New York. Jean Baptiste LaPorte also lived there:
Jean Baptiste (based on some research that I've done and that I've borrowed) appears to the be "John" listed here in Edward LaPorte's home in Beekmantown. Francis Nelson LaPorte was also in Beekmantown in 1850, but had moved to Mooers, New York by the time of the 1860 Census.
The final thing that has sparked my theory that he is Jean Baptiste's son, not Charles's son is the intertwining of Francis Nelson's children (particularly Sidney Nelson and his family) with another LaPorte family -- Joseph LaPorte (who married Nancy Beagle). Sidney Nelson LaPorte had some inter-connections with the LaPorte-Niles group that later produced a number of Niles-LaPorte intermarriages.
A death record for Francis Nelson LaPorte in Beekmantown would be a nice thing to have . . .. Also, I need to re-examine all of the "evidence" that I had for initially forming the Julius/Julien/Francis Nelson are brothers hypothesis. I wonder if it is any deeper than a forum post that I saw years ago.