>>> I'm worried I may be hurting my sons swing more than helping. I've had him do bag work and accomplish those goals before moving on to hitting a ball. I let him hit off a tee this week and it's like he never hit a ball before. Looks to be spinning and stopping at the ball on contact with a terrible follow through. Like he doesn't know what to do post contact. ugh.
Think I may be in over my head here and messing with him more than just letting him swing naturally. <<<
Sounds like your son indeed does have a problem. His bat should be accelerating at contact. If the bag were not there to stop the bat, the bat's momentum would pull his limbs and body through the follow-through. His bat stopping at contact indicates it is decelerating approaching contact.
The "follow through" phase of the swing occurs after contact and the ball has left the bat. We must keep in mind that during the bat/ball collision, the bat is only in contact with the ball for 1/2000 sec and the bat only moves forward about 3/4 inch before the ball is gone. Once the ball is gone, continuing to expend energy is just wasted energy that should have been more efficiently used earlier in the swing.
Great hitters have swing mechanics that generates acceleration right from initiation and attains maximum velocity approaching contact. The way great hitters depleted their rotational energy by contact is similar to the way a lumberjack sinks his axe into a tree. The lumberjack depletes his energy accelerating the axe. It is the momentum of the axe that sinks the blade into the tree -- not the driving of the arms. The lumberjack's arms and hands are almost motionless at contact. -- If the tree were not there to stop the axe, its momentum would pull his arms to extension during the follow through.
I have not encountered the problem you describe and without seeing your son's bag-practice I cannot comment on why his mechanics produced the results you describe. I will place below a link that describes the correct use of a heavy bag.
Confusion/heavy bag drill