While 3xBB is my standard PFR, when the blinds are only 10/15 I usually raise to 90. Anything less doesn't seem to thin the herd out too much.

His bets seem to me that he's more concerned about keeping you in the pot than anything else. Small bet at the flop and then calling your raise could mean that he flopped a straight or set and is just trying to milk you. He's also getting the right pot odds for a flush draw by calling your raise at the flop.

When he checked the Ace at the turn and then called your 300, again it seems like he's more afraid of you getting scared off by the Ace than worried that you might have one.

His bet on the river looks like he's trying to squeeze more chips out of you with a hand he made at the flop, or less likely he just made his flush. With the possibility for a flush and a straight out there, I don't think your single pair will hold. I'd lay it down.