[leyhal]

He limps preflop.

He checks behind two players on the flop. Most players would bet 2-pair here, but many $10 players would drag a set. So he could have a set, an A-5 type hand or a straight draw.

He calls a PSB on the turn after a 2nd spade hits. This could mean he turned a flush draw to go with his A-5 type hand, or he possibly hit a straight (but straight much less likely as you hold 76). A set is less likely if he is any way savvy because board is draw heavy and most players would raise the turn to protect his hand and get value. I think it's a pair + flush draw or a made straight, and a lesser possibility of a set.

River: The flush card comes in and also pairs the board. So that leaves a flush, straight or less likely full house. If he hit the flush it is a back door flush, so he only started with 1 spade in his hand, like i said say As5s..

You are getting 2 to 1 on the river to call. Is he a bluffer, does he try to buy the pot with weak hands? It depends on the players tendencies. What would he push with that you're beating? If I knew nothing about him I would fold because
1)he showed no strength up until the river
2)he must reckon you have a good hand seeing you bet the turn and the river
3)The board is very scary to bluff into

so unless he is a complete idiot he must know that you likely have a strong made hand, at least a straight.

If I knew he was a bad bluffer I would call because the fact the flush is backdoor makes it far less likely and bad bluffers love to bet these type of boards.