He could be playing the nut straight there but there's really many other hands that would play the same way. I would be tempted to fold if I had a read on him, but generally I would call and see the river. You have 10 outs for the boat and I would likely call regardless. If you do make the the 10 outs and he is holding the nut straight you will take his whole stack based on his play there. Taken as a whole I think calling is positive.