We have a player who has
A - Little or no hip rotation in his swing.
B - Scrunches his shoulders up and never really gets full extension on his swing.
I feel that a causes b to happen.
Because he only has his upper body functioning he's working twice as hard to get the upper body rotating.
I'm getting him to work on his hip explosion, which i feel will make it easier to work on getting the upper body hands drive the butt of the bat at the ball and make getting to full extension
Our other coach is working on his upper body mechanics first. I know both a and b have to be corrected. But is it not better to correct a then move onto b. Don't want to confuse the kid. What do you think?

I would be inclined to work on A first. But I also work with a hitting coach who I'm certain would be more about hands and upper body drive.
Sometimes it comes down to which way a player will learn best - and unfortunately you can't know that until you try.
That's not a cop out (well maybe it is) but that's why I want to leave this question posted here - to see what other comments we receive over time.

First of all you have I am assuming we are working with a younger player. First thing I would do is get the scrunch out of his shoulders, you can't swing if your muscles are tight. Start the session out with a little fun, put a basketball on a tee and tell him it's a bee and swat it as hard as he can, laugh with him get him loose swinging hard, some coaches approaches are stoical, create a fun enviroment. Once this is accomplihed I would then go down stairs and work on a comfortable stance, balance is key, but can be different for each player. So work on that with his input and find which stance best works for him, give him a little shove and see if he maintains good balance. From here with my younger players I place the bat in the crook of the elbows behind their backs, have them take a few swings and then i place a hand on their shoulder and one on the head of the bat, have them swing, applying just enough pressure to really make them use their hips, the swing should take about 3 seconds. During this process the focus is not only on working the hips, but to see if they maintain balance, some will try to lean back thinking it adds power, but it just gets them off balance, correct this and keep going until you comfortable they are firing their hips with good balance. I would now work on loading the back side, there are many ways to do this, from just lifting the front foot transfering the weight to the back foot, striding out 6-12 inches or no stride at all, the goal of all them is to transfer 70-80% of the body weight to the back leg. Use what ever is most comfortable to the player. I see that your approach to hitting is more linear, driving the butt of the bat at the ball, which is fine, but I have found for my younger players it's easier to teach them a more rational approach and I especially like Coach Petricca approach with torque hitting, he kind of connects all the dots with the bat on the shoulder approach, which forces players (especially young ones starting out) to use their bodies more. You know teaching baseball is a lot like my other passion fishing, load up the tackle box with lots of lures, if one don't work the next one may. I say this because depending on theory hands and upper body drive may be different, Web ball does a nice job of explaining the different theories and I would recommend knowing as much as you can about them. Sometimes you may even come up with a hybird of rotational & linear. Like Rich says above, "sometimes it comes down to which way a player will learn best". Good luck, Coach Moore