I think Consanguinity will make the strongest son the heir. If so, maybe that would do what you want.
Other than hating your oldest son is there any special reason you don't want him to inherit? Bad traits? Crummy stats?
When you fought and defeated him why didn't you revassalize him?
Good luck vs. HRE. If they have a strong and united kingdom, as happens occasionally, they will be painfully tough to fight.
Well, he's certainly a jerk for defying my divine authority, but I was more concerned that since he defected to England, if he inherits, I'll lose the game. He rebelled with his county, and surrendered the title back to me within a month or two, which I thought would mean he re-entered my court as a courtier, but instead he defected. Sometime along the way he gained the trait "rebellious" so I'm not sure if that's cause or effect.
My worry about consanguinity is that I think he IS the strongest, but is sworn to a new liege. His other brothers are quite weak.
Since he offered peace terms, I didn't see an option to re-vassalize him, and like I said, I assumed he'd "come home", so to speak.
The HRE war could go either way at the moment. They seem pretty well organized, but a fair fight is at least something I understand. This succession stuff is a bit ambiguous (but much more interesting).