Seeing that it's 25NL on PokerStars
, I'm going the opposite way. It being only .55 more to the big stack in the big blind means he could have anything. Ace-rag, suited connectors, or any 2 suiteds could all be possibilites.
His overbet OOP on the flop should mean something. I'd say he has a good hand like TPTK, 2 pair, an overpair, or a set, but not the flush. So he is banking that you didn't flop a flush and wants to discourage you from chasing the flush with one high spade.
Now the exact same betsize on the turn means he's either betting for value or has a second-best type hand and wants to look strong at lower risk. I'd say that if he had something worthless and was scared, he'd either check the turn or overbet again.
Though 66, 55, 44, 77, or something like A7 are likely, that price on the turn is cheap. So I'd call down and hope I have him beat. Most 25NL players play OOP pretty poorly, so you have a good chance that your set of 3's is the best hand.