I would look at it this way: when he plays the river this way, it is almost always a bluff. For him to check jam this river, he is representing a really strong hand, at least 4x.

If he is that strong, he is never ever ever checking the river. The SB already checked. You cbet the flop and then checked the turn, he has no reason at all to think you're going to bet the river.

If he has a strong hand, he is betting the river once the SB checks damn near 100% of the time.

He just knows that you're hand looks weak and the SB can never be strong once he check calls the river, so it appears like a good spot for him to bluff, except he represents absolutely nothing of value.