His "front leg open" as you describe it is really a "good" thing as that is key to having his hips release in a rotational manner. If you try to eliminate or restrict opening that foot/leg, he'll restrict his follow through and lose power. After his first couple of swings I like his position at contact (see :19), though I might like to see a little more flex in his knees.
The stand out problems I see are twofold and one is what Low Finish mentioned. There's no "coiling" or tension being put into the upper body. I sometimes refer to this as loading of the shoulders and hips. You're son's upper body isn't really doing much as his strong arms are doing all the upper body work. The power that he's able to generate is coming mostly from his lower body.
So the second part I see would have to do with what is referred to as "separation". Separation is the difference in the angle between the vertical plane of the shoulders and the vertical plane of the hips. Your son isn't getting any separation at all as both shoulders and hips move together in the same plane when initiating his swing. Let me try to illustrate - think in terms of a long square peg made of rubber standing on end. Hold the bottom still while giving the top a little twist and then let go. Note one of the flat surfaces as you twist where you see the surface facing different directions along its length. The difference in direction is "separation." Basically, it's the same thing in pitching, though hitters can't produce or use as much as pitchers tend to generate due to having to react so quickly.
Go on the internet and take a look as some professional player's swings and see if you can see the coil and separation and compare it to what you're son is doing in this video.
If he can do this "coiling" and stay balanced with some flex in the knees, then I think he'll LIKE the results.