Yeah - Villain is one of those donks who doesn't have very much conception of betting percentages of the pot. So, this could mean a couple of things: a. he missed his flush on the turn and is not scared; b. He hit his flush on the flop and is now playing his hand super goofy.

I would raise to 35-40, if he re-raises you all in, fold and save yourself the money. If he flat calls, chances are he does something monstruously idiotic whether he hits his river or not, like betting 20 into 100, and you can call quite satisfied to know that you only have to be right ~21% of the time to profit. It's kind of a win/win situation here.