well, the fact that he is overbeting every street either mean he's bluffing or he that he holds the nuts. I guess this calldown is fairly good, but not extremly impressive.

I would imagine that you have played more than 500 hands with the guy and probably you haven't seen him overbet alot. Thus, you see him donkbet into you and you realize that he must be on a draw or a very strong hand. When the king comes he doesn't even think, he overbets again, and you think: Its even more obvious that he either has a draw or a very strong hand considering that he didn't even think when the king came. Normaly the king would ruin his hand. Due to the donkfactor so far of your opponents play, you decide to call. The river blanks, and now your opponent has a big problem. He has NOTHING! Now he is thinking for like 30s, and then he leads out overbet once more. You think for 10s and realize. This player clearly has not the king, he wouldn't play this way for value with a 9 so: either he is stone cold bluffing or he has cought a set on the flop. Considering his donkline and the fact that you haven't really seen him lead out overbet 3 streets before, you feel rather confident that he is actually bluffing here more often then not!

Obvious call on the river, when you have called both flop and turn!