You don't like defending the hook with your hand, but trust me... it'll hurt a lot more if the hook lands flush on your unprotected chin. Against a known hooker, keep your right hand by your head as you throw jabs (I'm assuming you're both right-handers). Also, do some circling to your left. Takes away much of the power of the hook. Many times (not all) the hook is the second punch in a two-punch combination where the right is not meant to be the knockout punch. Avoid using your right hand to parry his right hand... the hard left hook is sure to follow. Finally, be wary of the distance and vary it often. Don't keep the same distance too long, as it will give him a chance to calibrate the arc of the hook.