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Much of this "strength" that is needed for hitting comes from core strength and lower body strength. Training your core and focusing on rotational core exercises and lower body strength will greatly increase hitting for power and driving baseballs instead of just hitting a few hard ones now and again.
About your son hitting outside many times, it has to do with the location of the ball in the stance at contact. Many balls that are hit oppo are hit inside the stride foot towards the middle third of your batting stance. Inside pitches that are "inside-outed" to the opposite field are still hit deeper in the stance than if you were to pull the ball. Tee work is the best way to teach pulling a baseball, although many times it is the other way around(teaching opposite field hitting). Set the tee up for an inside pitch with contact being made in front of the stride foot.
About your son hitting outside many times, it has to do with the location of the ball in the stance at contact. Many balls that are hit oppo are hit inside the stride foot towards the middle third of your batting stance. Inside pitches that are "inside-outed" to the opposite field are still hit deeper in the stance than if you were to pull the ball. Tee work is the best way to teach pulling a baseball, although many times it is the other way around(teaching opposite field hitting). Set the tee up for an inside pitch with contact being made in front of the stride foot.
