He seems to have flopped a set in here. It is very plausible to me that he did not raise pre flop with 77 or 44 (perhaps with 99 also). I this situation, he calls your raise to see what happens (if he flops a set). I think he would raise PF with JJ+ and would not call your bet with 4-7, 9-7 and 9-4 (two pairs).

He would not reraise you on the flop with a draw...at least, he should not. Any kind of bluff would be detected after your all in bet...its an easy hand to fold if you dont have a good hand.

But he called.