I think you need to raise the bet on the flop. By just calling you have no idea where you stand. If you raise and he comes over the top you can still get away from it if you are assuming he has the 6 (you said it was a loose game) or pocket 8s. More than likely he's on a FD/SD and you either make him pay for his draw or you win the hand right there.

Plus his $20 bet is pretty weak - it is less than 1/2 the pot so he still hasn't shown any real strength.