Well, I was very into History as a kid and used to listen to the stories my grandad told me about the war etc. And that was before I was ten.

You can answer the questions say about WW1 and 2 easily enough. Why it happened, who was involved etc without going into too great a detail.

Also, get him some kids books on ancient wars.

There is a great series called World at War which covered the whole of WW2. If you obtained them, you could watch some with him maybe.

http://en.wikipedia.org/wiki/The_World_at_War