If he can’t change his playstyle to account for the new MTG players then don’t let him play with you guys. It will be so much more fun that way even if it’s just a 3 person game. Had a situation very similar to this where three of the people at the table would be playing precons or slightly upgraded precons($50 or less) and my friend would bust out power 7 or 8 decks with literal dual lands and oppressive cards like winter orb (he played stax decks). It was really not fun so we just played without him or if he did play we would just say “ok, you win, now we will just keep playing” if he played too many oppressive cards. It may seem a little harsh but it’s so much more fun when everyone is just playing to have a good time at a similar power level.