How is his hip internal and external rotation from left to right?
How is his ankle total range of motion from left to right?
Have you cued him to drive his knees out or put a miniband around his knees during a squat to force him to drive the knees out?
Who says you have to squat to get better?
Unilateral work is your friend.
Your top 3:
Barbell Reverse Lunge w/ Front Squat grip
Bulgarian Split Squat
Sled Work (forward, backward, sideways)
In addition, you must absolutely strengthen the lateral hip and external rotators. Gluteus medius, minimus and maximus, TFL.
Monster Walks and mini-band side shuffles should be staples in every workout or part of his daily warm-up.
Foam Roll the quads, adductors, abductors, calves and thoracic spine each day.
If it's a structural thing maybe there is less he can do, but it probably isn't. Still worth getting it checked out by a specialist, but don't even think about surgery if he is functional and not in pain. You can accomplish everything you want in life without squatting and a lot of imbalances seem to go away after following a well structured program for a while.