[huhwhatyousay?]

Bet $4 on flop for value and to minimize playback. I don't see how a 18/15 has many hands such as A4ss and 98ss in his range cold-calling from the SB. So with the description you gave, I'm pretty sure you're WB the majority of his range and flipping with a negligible portion provided he never c/r hands like 99 here. Fold.