Coach May,
Personally I think there are some people who really know hitting who post on here. Myself I need to see a hitter completely before making suggestions. I can't see everything I need by watching video. However, I do think video is one of the absolute most valuable things for studying the swing. However, I like to see the whole thing, the pitch, the location, the timing, etc. in addition to the swing.
Your son sounds like my son who I mentioned before. I didn't tinker much with his swing, but we did talk about hitting. We had him think opposite field on everything except fastball. This didn't mean he actually went opposite field, but his thought process was opposite field. It worked well for him. I think it's the mental process more than the practice. If you can train your mind to think opposite field on everything except fastball, you tend to stay back longer and actually swing at those pitches less often before two strikes.
I thought my son hit the better pitching best also, but it was really just the better of the not so great pitching. Fastballs at 90+ he hit very well, breaking balls and off speed he hit OK. But there comes a time if you play long enough when the breaking balls are much sharper with more velocity, and the 90+ fastballs are better with more movement. That is when the faults in the swing become magnified.
By the way my son was a straight up narrow stanced guy. I never liked it, but the results were outstanding. His actual swing was beautiful with great bat speed. He just got to the level where he couldn't hit certain pitches very well and those were the pitches the best hitters need to hit. Mainly the good running or sinking fastball.
There's 90+ mph fastballs that are straight and there are those that are not straight. (I'm not telling anything everyone doesn't already know)I really believe that if he would have developed better hitting mechanics, he had great hitting skills, he might not of been quite as good early on, but more prepared when he got to professional baseball.
Definitely not saying this is true in all cases! There will always be exceptions to the rule.