My suggestion is to simply accede to his wishes. Support him and his chosen and be gracious and kind to them both. Yes you can bring up the Catholic conventional wisdom but don’t force anyone.
The reason I say this is if they are planning to have children, it is often then that adults/parents again expose themselves to the Catholic faith, by realizing that children need some foundation. I’ve seen many parents who don’t practice themselves, bring their young children to CCD and Mass to give the child a grounding in a faith. Much of the reason is that the child needs exposure now and then can make up their own minds when they are 18. And this exposure of little ones is a perfect opportunity to do some faith formation work with the parents that they just might have been deprived of when the were kids.
I guess I’m saying, save the “fight” for a better/future time. Live today, fight tomorrow"
My $.02