Your mom is wrong, and that's a terrible thing to say.
He doesn't want to talk about it. He doesn't know how to talk about it. Do not take it personally.
He's probably not eating. He's not feeling alive, in fact he's feeling extremely dead inside. He is asking himself what is the point of life, because up until then, his dad was immortal and everyone thought the story would just go on and on forever. Life as he knew it is over; a clock has started counting down, and nothing matters because nothing will last.
If he strongly loved his dad, he is also overcome with survivor's guilt. He's wondering why is he alive, while his father, who deserves more to live, is gone. He's wishing he could somehow give his life so that his dad could come back. He's replaying in his mind all the things in the future his dad was supposed to witness and now never will.
He wants to know you're there for him but no more, there just isn't room for anything else. You might not have the tools to relate or connect, but he doesn't blame you for that nor expect you to.
This is the time you need to be a rock for him and not expect anything in return. Remind him you care but be solid. Avoid making up anything to help him feel better, like "I know your dad is in a better place" or "things will get better." These statements trigger a "how would you know/what would you know about it" and only make him feel more isolated because it tells him you don't understand. And finally, whatever you do, don't display distress because it puts the blame on him and doesn't help. Think of him as lost at sea and you're the lighthouse standing tall in the rain.
Sorry for the verbosity but you seem like a good person and I thought I could sort of vicariously communicate in his stead.
I would recommend keeping your interactions short and life-support related. Fix his food, drive him where he needs to go, hang out doing your own thing two rooms away, things like that. It can be an opportunity to show you understand and he'll remember it forever.
I sincerely wish you the best.