To be clear, I'm not advocating this as a standard play, this hand is far from standard and would qualify for a bad beat jackpot in many casinos
To me, his line is perfectly consistent w/ a flopped nut flush turned straight flush and not many other hands. What other hands was the OP thinking he could have had? A high flush like KQs, and 5s, 3s or 2s (did he float with an underpair and make a set on an ugly board that filled up on the river?) full are really your only hope. But, I feel like a high flush, KQs for example, would have bet the flop harder and bet the turn in case you were drawing to the nuts. The other hands AXs, and 66s had you crushed on the turn and can be made with more combos. (at best, 10 v. 15?)
The min raise is really helpful, esp if you know he's a fish. He flopped the nuts and doesn't know what to do with it so he just raises a small amount rather than a normal amount b/c in his head, a normal amount might cause you to fold all of your hands that are getting crushed w/ <5% equity or drawing dead to a lower flush, which makes up the vast majority of your range at that point. His flop bet was for "value" but was sized terribly.