What can I talk to him about?

Your day. It's simple, and that's what it breaks down to in the end. If you start by asking him what he did today, even if he gives a simple answer, he'll at least ask you what you did, and that will open you up to tell him. Give him some details, but not a lot and hopefully something that one of you says will spark something that the other one is thinking about. Never ever ask what he's thinking about though, its a wast of time, and he'll always say either "nothing" or some variation of that, and he'll get annoyed at you.