Or he is trying to get a cheap showdown with top two (or one) now that he sees the board pairing has counterfeited his hand.

A big % of the pots AA wins are from the board pairing small on the river and counterfeiting someone's flopped two pair. Just calling here is the right play; certainly you can't fold, and raising is only right if you can give the opponent a range of hands that will call but still lose. I don't think that range is there.