It’s normal for the hips to start to open right before front foot plant that is usually necessary to allow the front leg/foot to open up into foot plant. How much the hips open up before foot plant depends on the pitcher’s flexibility.
My thoughts are that the HS coach should never have messed with your son’s “closedness”. If your son’s stride length was too short, that’s a separate issue that should have been worked on separately. But, having said that, I’m a firm believer that stride length is a result of other things earlier in the delivery so you don’t directly work on stride length - you work on those other things.
To get away from the front leg reach, consider using the Hershiser drill along with the Cross-over drill.