Yes, wait for him to mention it, that kind of issue is something that involves extremely personal emotions he may not feel ready to share with you now. If he brings up that his mom has passed away then don't feign surprise, be honest and open. He won't be angry because you didn't bring it up with him when you found out, just be prepared to listen if he ever feels the need to talk about it.
I have seen the mystics play there,
Once or twice but I knew they had a reason.
Enchantment plays it's cards all right,
Hand in hand with the working of the seasons.