My daughter & her husband say he never mentions Papa around them. He hears enough of what's going on and feels the tension, so he just accepted Papa is not living with Grandma anymore.
Here, he especially misses his Papa. Going fishing, etc., the things they did together when he stayed with us. He understands but he doesn't understand, you know? And I hate to blow it off as "He'll understand when he gets older"
We all agree that if he isn't going to be a presence in their lives, it's better than just popping in and out when it suits him.