I had one of the Age of Empires titles (II, I think). It's not terribly complicated. It's basically the original Warcraft game (not the MMORPG) in different packaging/context: gather resources -> build buildings -> generate fighters -> fight, all on a restricted-size game map that plunges you into conflict quickly. The only issue it would have for a younger kid is that you have to have a plan for getting your resources and basic defenses up and running quickly, as the AI likes to try an early swarm attack that's very successful against novice players.
In a similar vein, if he likes Age of Empires, I'd give the Command and Conquer titles a shot. Same sort of game in a modern warfare context, but it stands apart because of some pretty good story-telling, and a wide variety of challenging battle scenarios wrapped around those.
