He obviously has 25o and has hit his guthshot straight draw.

All jokes aside however, I would usually say he's hit his set. He was trying to limp into the pot with 44 or 66 and you raised him. He saw that you had a nice juicy stack to double up on if he hit his set (and also realized you are raising with goodness, yes goodness) and chose to call (though a questionable call). Flop comes...BOOM, SET! Get outta there! I had a hand similar to this yesterday (posted on my OP).