Murderate the guy.
Rebellions will happen eventually. If only to press claims on the kingdom. His vassal opinion is high now, but he can neither live forever nor keep on fighting. Plus mongols will come knocking asp, right? And china can always do its job.
In my own world, its 1000, almost, and my god Ive grown out of control. I took aquantine from the andlusian kingdom of usurped-the-uyammads but not the new emperor and managed to die in rapid succession. The last (good) king died of being murdered by his sister's random-ass and jealous husband, and his diplo was 28! The queen just died of bloody scurvy, and her son has had his hand chopped off and died of a really bad court physician. A duke has successfully fabbed a claim in the throne, and because none of the previous three or four kings of sicily have managed to establish a long reign or good reputation they seem ready to revolt against my child-king. Next in line is his cousin who lives in whats left of Lombardy and has admittedly good stats but I dont really kin losing my hereditary line of kings to be frank.
Plagues keep stalking the kingdom specifically, and Mongols are currently rampaging around in syria. They've managed to take southern parts of byzantine, and OMFG THE SECRET CULT OF ICONOCLASTS HAS CONVERTED THE MUSSIE OF PERSIAN EMPIRE AND THE DAMN KHAN. for reals, the khan of mongols has publicly gone iconoclast and the Caliph of Abyysasid has secretly adopted it. At least that was the case when my little imbred duke was floating around . . .
This iconcolast cult is proof that the illuminate exists. Its literally worldwide atp. Funnily, the tengri faith was reformed prior to this. German reliigion is sticking to its roots up north despite their reformation, and the world is actually kind of boring in its relative inaction. Things seem to follow a relative status quo around. Except the mongols, oc