My suggestion is to abandon the balance point idea. I know coaches still teach the balance point but fact is that many of the top pro pitchers never reach a balance point. That means they never get to a position where they could hesitate in a balanced position. They certainly have no hesitation. Take a look at the clip of Randy Johnson playing at the bottom of this page and you'll see his hips moving forward well before the peak of his knee lift. There's definitely no point in there where he could pause in a balanced position. And there's definitely no pause anywhere.
I agree with this though I'd say he needs to be over the front leg sooner - at release - not later during follow through. He can make this happen by getting his front hip moving forward faster to build more momentum.
I disagree with this. I think he should be able to get himself out over the front leg and retain the stride he has. In fact, in the video he appears to spin out of his delivery and fall off to the side so I could argue that he's not striding as far as he could/should. But I'd say work on building momentum and let the stride happen.
Getting the front hip moving faster to build more momentum will pull the back leg around.
Opening up early IS a timing problem. His spinning out of his delivery and falling off to the side is evidence of this. This will also be fixed by getting the front hip moving forward faster.
I think your son is very astute in that he has identified the symptoms of his problem. I feel the cause of these symptoms is the lack of momentum. Have him get the front hip moving toward home plate faster and possibly sooner. Make sure he leads with the front hip. The head and shoulders stay slightly behind the front hip into foot plant but should not lean back much - good posture must be maintained. Also make sure he doesn't sacrifice his knee lift. Instead, he needs to force his leg to be quicker from knee lift to foot plant. A good checkpoint I like to use is to get the front foot, knee, and chin pretty much aligned vertically at release with the release point about 8"-12" in front of the front foot.