I completely agree with Sue, you need to let him know that you care about him no matter what. I lost my father when I was 12... If he wants to talk about it, show him that you are open to it and are not uncomfortable. For example, when I was with the rest of my family, what I hated the most was when people would swiftly change the subject when someone mentioned anything even remotely related with my father, and then they would all look at me as if checking that I was still ok... It made me feel like talking about him was wrong. Back then, I think I'd hate if anyone forced me to talk about his death, but at the same time almost no one talked about him as a person.
Now, 14 years later (and after I have also lost my mother) I realise how much it would have helped me if someone had just talked to me about my father as a normal person, with memories, with funny stories in his childhood, etc, instead of pretending he hadn't even existed.
All the best for you and your nephew*