Depending on how hard he throws and how often, he could just plain be damaging his fingers ad his hand, never mind about learning to throw a hook.
If he needs to learn to bowl, he should see a good coach.
Also, if he doesn't know how to throw a hook and he just got a finger tip ball drilled, he may just not know how to throw a bowling ball, how to seat his thumb and his fingers, how to hold the ball, hot to release it, etc.
Was the ball drilled by either an IBPSIA certified pro shop or by a good driller (that you are aware of)?