I used to squat a lot and I had the same issue. If I reduced the weight, I was able to perform a squat without my knees moving inward (towards each other). I varied my squat cycles to higher reps with less weight for awhile and added adductor specific exercises and it eventually balanced out. Your son is young and probably growing. Get a ball between his knees and perform a squat against a wall. Hold it with thighs parallel to the floor and squeeze the heck out of the ball (the ball squeeze works the adductors and the squeeze will also align the pelvic bone). This can be done on the floor as well. As your trainer for his plan to test for specific weaknesses he mentioned and his plan to fix and maintain going forward.