[soded]

I would just call.
People can call with weird stuff in the blinds.
In this case though, he's not betting very much, he's giving you 3.5 to 1 to call, which you will win more than 30% of the time. He might have just felt that you were weak after the turn check and that your flop bet was just a blind c-bet.
I think I would have bet the turn. If your 6's are good on flop, they're pretty much good when that 2 hits on the turn.
Also with VPIP 31 (although only with 15 hands, which doesn't mean much), SB could just be protecting his blinds with A4 or K4 or some other garbage... although A4 does have some showdown potential cuz of the A... Another factor to put him on a wider range, players tend to play much more losely short handed, they take the TAG play and throw it out the window and become much more LAG, with the mentality that more hands will hold up against fewer players. This is just a generalization, but at these limits, it's not hard to find such players.