Yeah agree that bet sizing is the main problem here. Firstly, I'd raise 4bb pre, but I don't have as much of a problem with pre as I do with raising .40 into a .75 pot on the flop. It kind of throws me in terms of hand analysis, because your bet gives less information about villain than a say $0.60 bet.

As for his range, I'd say remove all 2p's as he prob wasn't calling with that crap. So we're left with sets, missed mid range PPs, overpairs, AT, or perhaps the entire of the rest of his opening range cos he thinks the small bet is a weak bluff.

That is not to say that betting this small to induce a bluff is a good idea, just that the flop bet makes your life so myuch harded in terms of trying to read villains hand.