would the back elbow coming first, would that cause grounders?
This is definition of bat drag. While you can extend your arms and pull HRs (in the cage), the bat head stays behind the hands longer, and it's most difficult to hit outside pitches with any authority, and you get weak oppo grounders.
Uncoiling/opening the hips before toe touch makes the back elbow drop, which leads to bat drag. Essentially you begin your swing in mid-air on one foot. [notice that he coils inward, but he's over his back foot. Then when he starts to move forward and he opens his hips]
To stride correctly, you must have a flat/horizontal rear forearm at toe touch. [it makes no difference where it begins in the stance]
How to fix (isolate the stride without a ball or tee):
Get in front of a mirror and take your stride, and hold a 45 degree inward coil (hip and shoulder) until landing. Do this for a few days until you get the hang of it.
You don't need to try to 'sit' or 'set' or 'crimp' your hip, or pick your front hip up in the air, or have a certain leg kick, just rotate your torso - it's simple.
Tip: Keep the back heel on the ground as you stride using the back leg. Try not to turn your front knee over/forward until toe touch.
Then add pulling the elbow back like a 'bow'. Then make sure the rear forearm is flat at landing.
You and son can learn this in a few days. No need to buy new bats, pay instructors, etc.