Unfortunately, the crown has good relations with the Vallière family and a much greater need to improve its relations with its neighbors.
Ah, but don't forget that this is what, near-to-fore Dark Age France-expy. Intermarriage between the Crown and the powerful houses was so bad everyone was practically related. The Vallière are, from what I remember of Canon, one of the most powerful houses in the kingdom, and one of the very few with both the relations and position to make for a suitable "match" for Henrietta. Also, being the second-born son Shade is in the unique position of being an ideal candidate, since Eleonore is set to inherit the Dukedom, making him a very valuable bartering piece as the vaunted Second Son.
Politically speaking, that makes him ideal for the position of King-Consort, since not only does he have the right pedigree, his marriage basically ensures the loyalty of a large, powerful noble house, one that's only a few steps away from being royalty themselves. And that's not even talking about what the Vallière family might be able to leverage as a Dowry from the crown, including but not limited to political favors, extra rights or privileges, outright wealth and so on. Because, and don't get me wrong, I'm sure the Duke and Duchess both love all their children, this is one of the most politically cutthroat time periods in history. In no world would they ignore the potential reward for pressuring a Crown marriage to their strong, healthy, noble son as soon as they were able. You don't get the kind of power they have by ignoring such an opportunity.
I feel for Shade though, man. This is the Shit-Ages. If he does get betrothed, and all the nasty implications it might imply, he's going to have thirteen, maybe sixteen if he can finagle it, good years before he's required to elope, especially given Henrietta's mom's degradation once the King dies. The pressure for there to be an heir to the throne is going to skyrocket once that happens, and once the whole Reconquista thing goes off, that demand is going to increase by an order of magnitude at the least. Shade, I hope you got your Realpolitik on, 'cuz your gonna need it.