Play a series of games where they have all their pieces but you have only K, Q and the pawns. Your child will learn to enjoy chasing the Q around the board and will become more aggressive. Be prepared to lose lots of games but win when you can, don't throw any of the games.
See what happens.
I agree with Ghost; first that many very good players have defensive styles, and; second, that it is too early to talk about style. Also just let him enjoy it the way he does.