honestly, this is something more drastic of a situation than we think, we are talking about a stubborn man who has done surface level research on competitive (because he doesn't care about it), has only experienced A 6v6 valve comp match and deemed it bad and never played it again, he always said "TF2 was always made with casual in mind" which is not true but fully believes that, and has witnessed quickplay being taken away from him and only watched (after mym) as the comp players going back to the community 6s, keeping the weapons banned (mostly just base jumper now) even after the nerfs on the weapons. (which is understandable).
All he has seen and experienced is bad along side other games that tried to go for the E-Sport route just going to shit, dying, becoming unfun overtime, or just see toxic people come out because of competitive. He has never seen the bright side of it like, honing your skills, feeling more proud as you win a championship, making friends that push you to your limits, etc.
with the new video about "valve catering to competitive" coming in October I'm sure he has already made up his mind and won't change let alone agree to what you have to say when it comes to misconceptions.
if you are gonna discuss with him about this tho, try and find common ground first and talk him through step by step about each topic, but he might just not take it seriously at all and just try and make you look like the fool.
my solution, is to have an accessable 6s newbie only lobby (only accessible in newbie mixes or TF2CC) with the meta being the main thing but let players play full time offclassing, let them experience the flaws of the classes, that is the best way to teach someone instead of explaining it to them.