Our rich nepo baby friend sure does come off as an absolute failure. Sheesh, the poor father. Dude's at war for half a decade and his kid just lays about skirting his responsibilities. Then he funds his education and his kid spends years just talking hot garbage while laying around doing nothing. So he gives the guy an ultimatum: prove to me you can defend yourself and your people, as your position demands, or I'm finding you a wife who can do so in your place. And the little shit has to let himself by dragged back to your house, outright breaking the one condition you set, and has the gall to try and look you dead into the windows of your soul and lie about already having a fiancé.
If he takes control as heir their lands, their peoples, it's all fucked. They'll stand no chance under a lord who can't even handle marching alongside his soldiers.