Be yourself and say what you think is right. He may or may not want to talk about her for a long time but I would express some sentiment of care . It might be hard for him to bring up, but by mentioning it to you it will surely be on his mind. You could even just say that you know it must be on his mind and if he wants to talk you are there to listen. That way you aren't pressuring him but aren't ignoring it either.