If he can get by without casting spells either he's a fantastic roleplayer or your game is much too easy. Give him challenges that you think are impossible and watch him explore those challenges.

Or, play a different system. I'm going to go a different route than some earlier posters and suggest that it's okay to play a rules heavy system. You just need rules that focus on what he's interested in, namely interacting with NPCs. D&D focuses on combat and streamlines NPC interaction. I saw FATE mentioned earlier and that's a great choice. I hear Burning Wheel has an awesome social interaction mechanic, but I've never tried it. Legend also has a cool mechanic, which supposedly can be plucked out and inserted in another game, but again I have no personal experience with it.