[Bianculli]

I always assume an unkown player does not make this kind of bluff.

I agree with raising preflop. I would fold turn as played up to that point and quickly fold river as played up to that point.

His range is hard to say since he's an unknown. Definitely could be a boat that is hoping you had top pair and made trips on the river. He could have a straight in similar fashion. He could have a nine. He could be badly misplaying aa (which still has you beat). He could be bluffing, but like I said, I would never put an unknown on that bluff.