I have seen the film a couple of times and read the book, but I still can't figure out if he knew or not. Did he know and deliberately neglect her at the ball? Or he didn't know so he asked Becky to run away with him?
In the book I doubt it, as they had only been married 6 weeks and she didn't seem to know yet. In the movie yes--Becky sees Amelia is sick when she is dancing with George and asks if he shouldn't take more care of her now, he says something like "you mean now there is yet another rope to bind me" (to the marriage he's already regretting), the rope he means is the forthcoming child.
Come, we must press against the tide of naughtiness. Mind your step.