Unfortunately, no. There hasn't been console commands available in Total War since the change to TWEngine3.

The best way might be to force vassalize and annex him. If he's equipped any of his generals with it, you can just remove it once they're part of your faction and then fire them if you don't want the generals. Be warned that annexation does not transfer items that are in the faction treasury, but the AI usually equips everything it has anyways, so this shouldn't be a huge problem.