It's hard to imagine him bluff raising this turn card no matter how small you make your bet. I think overpairs would've raised this flop since it's wet as fuck and you donked (looks like a draw, at least in my experience) and he's not suddenly gonna decide to bluffraise when the 4card straight hits. Typically here i would risk checking back the turn to try and get him to bluff or herocall a river bet. I'd bet 2/3rds pot on all non-heart rivers if it got checked through or c/c on a heart. Hard to extract much from spots like this tho when OOP, that's why position is so sexytime.