If you let him continue "Your" story line on his own, then the next time you DM, think of an explanation to inform your players why his section never happened. Time Paradox? Dreamscape? There's lots of wonderfully creative ways to pull the game back where you want it without completely denying his style of play. Your best bet is, to start off continuing along his style and slowly moving everything in the proper direction IN game, so he doesn't feel like a boob, and your players aren't dissatisfied with you just saying "Ok so... that never happened, back to where we were." If however, he was DMing something completely different, then it's easy just to keep your two games as two different universes entirely