My only son is the heir to a city, so my titles will be inherited by my daughter. How to fix this?

Title kinda says it all - i have a son and a daughter, and he is the heir to some city i can't even find(it says city of York, and i have created a Republic from the Duchy of York, so there is Republic of York, Grand City of York and that's it - the city in the County of York is called something else - the title search finds it and sends me to the county, but there's nothing named City of York there).

What can i do to change this? I don't want my vassals to have the -10 modifier for having a female heir, and i don't want the little girl inheriting(i am ~40 years old, i might die soon, and she's 2), my rule isn't all that stable(I am Richard de Normandie, second son of William the Bastard, and the first to rule after William's death, and altough i managed to replace most of the Anglo-Saxon vassals with Normans, as well as remove most threats - all of the Godwins i could get my hands to are bishops or dead, save Harold, who is a castrated count under a powerful Norman Duke, and his biggest son, also a count under a powerful Norman Duke, same goes for all of the Hwicces, i don't want to risk it, especially with a strong French King with de jure claim on Normandy, ambitious Breton Petty Queen, whose husband and sister i have killed, and a Scottish King who hates me for taking one of his counties and executing some of his family).

So yes, how can i remove my son from the order of succession of a city? He's in my court, so i can send him to be tutored somewhere(in a few years, he's 4-5), or give him titles(altough i prefer not to, i like to have my family near me, so that i can control their actions[apart from when i send 3rd or 4th brothers to start a branch in some foreign County/Duchy/Kingdom]).