What sort of things could I say for comfort?

My friends son is 9 and he's sadly lost his dad recently. He has shown NO emotions what so ever and his mum is genuinely concerned. I'm very close to him and he's good with talking to me.

How do I approach him on this subject? As I'm the one he opens up to I was thinking playing Xbox with him and gently asking how he is feeling about things. I don't want to push it too much but I do want to see how he is feeling.

I mean we're talking about a 9 year old here. I've been personally asked if I could spend some time with him and maybe see how he is feeling. Again, what sort of things could I do or say? I just want him to know I'm here and so is everyone around him and he can talk to any of us!