I have two sons now, one is of my former husband’s side and older than the second son of my side from matrilineal marriage. The first son won’t leave the succession line. I can’t conspire to murder him since he’s my child.
What must I do to take him out and bring my second son forward?
EDIT. A bit of update, that stupid non-dynastic heir somehow inherited a petty kingdom nearby, meaning he was not of my court anymore, there was nothing I could do. I could not ask him to take the vows (since he’s not my courtier), couldn’t imprison him though I was willing to suffer the opinion penalty. I tried to fabricate claim and declare war with hope to capture him as POW but wasn’t a success. He took over the kingdom after my passing, leaving my dynasty to oblivion shame and putting an end to yet another noble house that couldn’t survive the feudal hardship.
There are 3 viable options:
Disqualify him from inheritance/holding titles, by making him a monk.
Change your succession laws, so the oldest child can’t inherit (Elective, Elective Gavelkind, Ultimogeniture, Seniority.
Kill (Imprison/Execute) him or blind/castrate him (If Greek)
You could also give titles to your prefered son, so you won’t trigger an immediate game-over if the non-dynastic son inherits your main title, and then, after you die, (the new) you can just press his claim.
Making your son a Commander and sending him into losing battles may also work out nicely. You could also use the Intrigue Focus to spy on him, and imprison/have him killed without plotting. Same for Martial Focus and Dueling if you’re capable.