During his lifetime, there was already speculation that he was fathered by Leopold II on account of the Belgian royal stipend he received throughout childhood, and I've read some compelling arguments that he might have been the illegitimate son of Emperor Maximilian and one of Charlotte's Mexican ladies in waiting. In any case, IOTL the mystery of his parents' identities was never solved on account of the people who knew the answer taking it with them to the grave.
But for this thought experiment, let us say that his royal parentage is publicly announced shortly after his graduation from cavalry school. IOTL, career advancement for devoutly Catholic, anti-secular cavalry officers like Weygand were poor, but if he had the benefit of royal prestige, could he have moved up faster in the ranks in spite of the Republican political bias against conservative officers?