I could say great swing! But that's not what you asked for- so here's my "internet" feedback- probably worth as much as the price you're paying.
You definitely want to have a vision of where you want the movement to be- in order to consistently guide you on your pursuit. The fact that he's not hitting his driver well now- isn't really a good reason to look for "what's" wrong. Anotherwords, it's not one tip here or another there- that will get you to where you need to be.
Minor points/observations
Driver stance looks a little wide, ball is extremely forward, and the toes of the feet appear to point not perpendicluar to the target line but angled "away" or behind the ball (to the right of the ball from the player's perspective)- possibly a subconscious adaptation to help prevent coming in "over the top"- or "too high and outside".
Major points/observations
The slow motion down the line sequences are the best to see the issues. The start down is not enough down, at impact the hands are not low enough and the right forearm is too high. Need more waistbend at setup, steeper hand swing during startdown, lower hands at impact, retain the waistbend through impact and into the follow-thru, which will help get the right forearm on plane. With a little more waist bend at set-up and a steeper start down while trying to direct the hands to a lower location through impact- I think you'll see a little better hip slide and the right heel rolling up sooner in the movement but that may take time to show up (just remember that the right heel motion would be an effect and not really something to work on directly). When you come down with the hands more out than down- the right hip doesn't have to clear and the right heel doesn't have to roll up as soon.
Great swing! (not Hit)
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Life Goal- Developing a new theory of movement based on Brain Science
Interests - Dabbling with insanity
Hobbies- Creating Quality