A few thoughts and suggestions for this situation in general:

Start with politely trying to work the player and discuss with them what they want and what you want. If the goals don't match, ask if they are willing to defect to a kingdom that is a better fit.

Failing that, then you need to kill him off.

Consider that intentionally staying small may leave you vulnerable to bigger players both internal renegades as well as outside provinces and kingdoms. Carefully examine why you aren't growing and decide if maybe you should try growing (this doesn't mean not warring.)

If the province grew fast, they probably have low wpa and possibly also have low tpa. Hopefully your kingdom has provinces who can take advantage of this. Removing his peasants is usually a good place to start since this will stop his growth and ability to train more military and thieves. If you've removed his peasants, you should be able to take them down over time with ops like MS, NS, NM, and then bounce razes until you can break on trad marches.

You can of course also ask other big kingdoms to hit him to speed the process.