Looks like he may have smooth-called the flop w/a big hand waiting for the turn to raise. Maybe 88 or 67s, as both of these are hands that a weak player might call w/first in; or something like A5s. Maybe he's being sneaky with a big pair, which is not too uncommon. The more I look at it the more confused I get, too. If he really had something big I don't see why he wouldn't cap the turn. My head hurts.