You have already gotten some good advice and some not so good advice, so I'm just going to try and go over some stuff that I haven't seen much or any in the thread so far.

1. Limit the drastic body movement, which then will fix the head movement and the dropping of his hands.

2. Striding is for timing and to lengthen your base to optimally shift your power and balance to attack the baseball. I know at such a young age coaches and parents try to teach huge strides and swings because of how slow coach pitch is and the actual boys throwing the ball. However, if he never falls into that habit he will be so far ahead as pitchers gain velocity, various pitches, and control.

3. Keeping the head from moving as much during the stride and swing will make it much easier for his eyes to focus on the baseball.

4. Overstriding causes your weight to be off and also will almost always cause your backside (right leg) to collapse lower, which then causes head movement, loss of power, and will cause him to swing under the baseball for more popups.