Thanks to all that played.
Here's what I had put him on: as Mad said, I thought he definitely could have had the set of 5's to flop an over set to my under set. That would leave me drawing to one out, the case 2.
However, the way he pushed me all in on the flop like that made me think that he perhaps had something like AK AQ suited, as the two was a spade on the flop and so was the 6 on the turn. He would be pushing with the nut flush draw and TPTK if he had AK, but my question then was why didn't he raise preflop if he really had big slick.
So, after trying to figure out if he had me beat, I decided that he didn't have the 3/4 and he probably didn't limp with AA either, so there was a good chance that I was in the lead or drawing to one out if he really did hold pocket 5's.
I made the call. He flipped A8 suited for the nut flush draw and top pair. River was 5 of clubs, filling me up, and his missed flush was irrelevant. I take down the pot.
But...I had such a hard time calling that bet right there. He had so many outs if I was ahead and he was drawing to the flush, or he could have the deck crippled with an overset. Very hard to play those pocket 2's sometimes, but it paid off big in this situation.
I also was big stack at the time, having his 42 covered by another 30 or so, so part of me wanted to forget the 15 I had invested in that pot, and pick a better spot. But the gambler in me, and knowing his tendancies, couldn't let me fold the hand.