I have no idea of his hand range since he is a relative unknown, I make the river fold since I am trying to assume my opponents actions are genuine until they prove to me that they can bluff.
I cannot imagine a player would call the turn with air and then check that river, I think a standard ABC opponent shows up with the
95% of the time here, since I am assuming they are an ABC opponent, I fold. Other made hands such as straights or baby flushes almost definitely check/call rather than check/raise.
For what its worth I don't like a bet on the flop multiway.