I’ll try to help you out be taking a stab at adding some insights to Steve’s comments:
1. Starting foot position on the mound
I’ll guess Steve is referring to your boy starting on the far left side of the rubber. Some coaches want righties on the right and lefties on the left to create angle. Don’t know if this is what Steve would suggest. Personally, I look at the effect starting position coupled with stride direction have on posture through the delivery and then adjust starting position to minimize postural shifts.
2. Lack of weight shift
It looks like your boy doesn’t get his center of mass moving forward until well after the peak of front knee lift. That’s generally considered late. The preference is to start moving forward before peak of knee lift. And I believe Steve agrees with this.
3. Hand break and arm action on leg lift
A common teach is to have the hands move up and down in sync with the front knee lift and then separate. Your boy’s hands move down as the front knee lifts and then hesitate while the knee drops. Hesitations in a delivery are usually frowned upon although it seems to be a common tactic used by the Japanese pitchers.
4. Back leg position before hand break
I’m not sure what Steve might be referring to.
5. Glove side arm action
It’s hard to see glove side arm action from the rear but it kind of looks like your boy might be pulling his glove back. If that happens before ball release, it usually results in early shoulder rotation which can affect performance and health. Not sure if this is what Steve is referring to.
Hope this helps.