So you believe a line drive swing should be taught until they are physically mature enough to learn to swing launch angle. I don't understand the logic in teaching two different swings based on a fallacy that youth players aren't strong enough to hit homeruns or whatever. Launch angle isn't about trying to hit homeruns - it's about getting the ball into the OF and use all that space to get hits. If it happens to go out then it goes out. No different than using a "line drive" swing.
Baseball is proportional so those youth league hitters who aren't strong enough to learn launch angle are playing against fielders who basically have the same amount of strength and skill. Teach launch angle from day one and it will provide the best opportunity for your son to be successful.