As a DM, you are also in a great position to encourage more roleplaying from him. Put him in situations where you try to draw social interaction out of him. When he does something well, make sure to reward him verbally - a simple "cool!", "man that was funny" or "great roleplaying!" can do wonders.
Positive reinforcement (and good rolemodels from your other players) can be an invaluable tool, when wielded expertly...
Positive reinforcement (and good rolemodels from your other players) can be an invaluable tool, when wielded expertly...