Gameplay tip? Son with 3 duchies makes "subjugation" war against other kingdom

[spotlight2001]

this is an achievement run for:

• Russkaya Pravda - As a Russian, hold the Empire of Russia.
• Lech, Czech, and Rus - Unite the Kingdoms of the three brothers, Lech, Czech and Rus, into one Slavic Union.

I play king of lithuania
unreformed romuva pagan
culture russian

I have distributed land to many kids (3 concubines + wife make lots of kids)
Unfotunately many kids died and now one son has 3 duchies

This son is about to win a CB "subjugation" war against Magyar.
I prefer that this 3 duchy son doesnt leave my realm before my guy becomes emperor.
How can I prevent this and preferably keeping the 3 duchy son alive and a friendly relationship

Tips?

I am thinking of a "revoke title plot", and fight his forces down a little bit.

 

[jonjowett]

I can't think of any quick way to prevent him from leaving. (You clearly don't have enough land to form an empire, and you don't want to kill him.)

My best suggestion is that you let him win his subjugation war, choose him as your heir, and then die asap. (You should have access to Eldership succession as unreformed Romuva, so you should be able to choose your successor relatively freely.)

Alternatively, you could let him win the subjugation war, which gives you strong claims on all of the land he rips out of your kingdom, and then you can press those claims. He'll hate you for it, and if you ruin his lands and armies then he might not hold on to his new kingdom. But this allows you to keep your kingdom's territorial integrity and allows you to choose a different heir (eg. a strong genius grandchild).