Well, yeah, it is after the fact, but he knows that you know he has a flush,

If the If that 4th card hadn't come off, and he still acted this way, I would agree.

Sometimes I make plays that look really weak, but I am correct often enough I think.

There are probably better examples of this guy being weak. I had some QJxx hand in an omaha tourney, I flop was JJx, I bet get called, turn is an A, I get get called, the river is a Q, I bet get called, the guy flips over AA77 - THAT is weak.