You don’t need to convince him to like Discourse, only to take his feedback on board, then figure out which are personal bias versus things which are genuinely detrimental to the community.
There are lots of large deployments which serve as great examples as to how much can be achieved though customisation. Have you shared any with him?
The quotes suggest a lot of vitriol, if he’s flatly refusing to consider Discourse you may be best taking him out of the decision making process entirely. By the sounds the decision is yours to make, which is hard enough without someone trying to hold you “ransom” if you go with something that may align more with your goals for the community.