Wait until he's standing with some friends. Let's say the group is made up of Bob, Tom, Jim, and Shame. You say, "Hey, any of you guys have living mothers? No, no, not you Shame." Or you can ask him, "Hey, how does it feel when your mother's dead?" Be subtle, because you wouldn't want to hurt whatever passes for his feelings, assuming he has any -- I've heard rumours that when his biology class pithed a frog they missed and got you-know-who.