Seems like we are beat at first glances, is he passive enough to check a set on the flop/just call on the turn with a set? Does he ever jam a 5 for value/as a bluff? Actually i highly doubt he would jam a 5 as a bluff based on the description you gave.
If he's aggro then its a harder decision imo, seems that this would be a weird spot for a passive fish to be ool. Is he ever doing this with a worse flush would be my real concern? He may think he's value shoving
or something.
As a side note, I think donking the flop is good here. Given your v.aggro image, you can expect calls from top pair + flush draw, or like Ksx all day long, and builds the pot up nicely for later streets. Especially if there is a passive fish who isn't putting much money in the middle, you need to start building a pot now (this is a concept Bart Hanson talks about in his podcasts quite alot if you haven't listened to them yet).
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