if he could check a flush on the river, he must think you can value bet non-huge hands there. if he thinks you can value bet non-huge hands there, there's enough chance for him to either c/r worse for value or do this insane bluff that i don't think you can fold. i get that it goes well beyond level 1 "i has a flush i can't fold" thinking. but if you consider his reasoning for checking the river to you to begin with, i don't think it makes sense to fold since, as flop and turn played, you're probably not showing up with bluffs often