[Ghazban]

He could have a ton of hands with the size of your preflop raise (to 4BB total after 3 limpers) and the expectation of a multiway pot. He could have flopped a set and is trying to build the pot. He could have 76s waiting to see if he could take it away on the turn if you checked a big pair other than aces. He could have nothing at all and be running a delayed bluff.. He could be doing a tricky turn semibluff raise with an OESD that just picked up a flush draw to go with it or something. In any case, it boils down to whether or not you believe him. You're clearly indicating you have at least an ace and he's clearly indicating that he can beat an ace.