I'm not sure "convert" is the right word here --- he is a Catholic, you are a Catholic, he just has some strange ideas about the nature of the Church in the present day, obviously fueled by the Dimonds.
In all of this, I don't think any of us are "written out of the Church", we just come to different conclusions, whether reluctant Summorum pontificuм adherents, SSPX, sedevacantists, home-aloners, Dimond adherents, or even if we've run off and followed "Pope Michael", or in the other direction, I would submit, those entirely doctrinally orthodox "conservative Novus Ordo" who think it is their bounden duty, binding under pain of mortal sin, to defend the Novus Ordo, Vatican II, and Francis and his bishops, to the moon and back. None of these people want to "go against the Church", they just have different ideas as to what that means and entails.
I would say just to point out his errors to him, make allowance for his disabilities, invoke the Holy Ghost, and let it go.