Well, he’s a Catholic (at least by baptism); it’s his opinion versus an Ecumenical Council. You could tell him that he isn’t given the option to disagree, but that wouldn’t work. Just explain to him what an "Ecumenical Council’ means, and if he still objects, you pray for him and then patiently break things down one piece at a time while the Holy Spirit does His work. Conversions don’t happen solely hrough intellectual arguments, and we aren’t the ones that convert anybody.
The modern Catholic Church doesn’t officially teach that all people are saved, or most, or few, or any other projection. It teaches that there is a Heaven and a hell. The 2nd Lateran Council, as well as every canonized mystic or Marian apparition that I have learned about thus far does teach that there are damned in hell at this moment. He shouldn’t be so worried about those that believe all are saved: it is an emotional opinion that is (in my opinion) divorced from the cardinal virtue of prudence, and the amount of material on his side eclipses the opposition. As far as the angels are concerned, there isn’t an option at all here: it is doctrine that devils exist, they have free will (just like humans) and that they are irrevocably damned.