You may find it really difficult to explain bitcoin to someone who does not have any idea about it. I think the best thing you can do is to ask him to read articles and research about it first on the internet. In that way he can explain how bitcoin works to himself. Then if he find something a bit confusing, entertain his question so you can clarify those. But still the best way to learn about bitcoin is to invest on bitcoin and learn from your experiences about it.