Well, I'm pretty sure he has a king in his hand, but I am not so sure about an 8,7, or 4 for the boat. If he did runner runner it, and limp in there with it UTG...he's pretty bad and buddy list him up. If he was betting out a set with the minimum on the flop, still a bad player. If he is a decent player, maybe he has KQ or KJ or something like that and thinks he has you outkicked.
I just call here though...if he was betting a set slow on the flop for the same reason you slowplayed the straight...