I like your little guy's intent.
When I first looked at your son I like the way he finished i.e. the intent that he had to throw the baseball.
He does get a little "funky" as he's releasing the ball. Funky in respect to he twists a little bit too much sideways.
One thing that's hugely important for you and anyone else who wants to have a fighting chance of coming close to maximizing a players abilities to throw the baseball, the head only follows a straight line if the player has a very flat delivery i.e. the elbow travels parallel to the ground as the player rotates as for the baseball. Some would interpret this as SideArm, but true side I'm is a different animal altogether as opposed to having a low elbow slot.
As the elbow slot gets higher i.e. as the player tilts his upper spine more toward first base if he is a right-hander and third base if he is a left-hander, the head will travel in more of a diagonal as viewed facing the pitcher from home plate.
When I saw your son finishing with his head moving toward first base the first thing I look for was a higher elbow slot which your son does not have.
And your son is not "pulling" his head off to the side as is his head moving off to the side so that he can get his shoulder around to throw the baseball.
Upon taking a closer look at the clip:
what did stand out to me is how he went into foot plant:
and more specifically:
He really hasn't started his rotation until after his foot plant but if you'll notice that his knee/foot are turned into quite a bit toward third base. and once he starts bearing up with weight he ends up with a situation where it appears that his front knee locks up and because it has nowhere else to go pushes his hips back and toward third base.
This creates all kinds of problems in trying to throw the baseball and he appears to compensate by hooking his spine around to the 1b side in an attempt to get things rotated so that he can throw the baseball.
High-level throwers rotate INTO foot plant. If you look at the Lincecom clip and for that matter Nolan Ryan their front knee has cleared i.e. is pointing toward home plate once the front foot lands.
Players who do land more close with the front side have learned how to rotate the middle section of their body to compensate for hips that might be a little too closed off. Some would call this The player having greater flexibility and it is to the degree that they have learned how to do it.
Obtaining proper hip rotation and hip to upper body separation is always a challenge especially for young players such as your son. That's what I would be working on.
Forget keeping the head online, that's another Tom House wives tale. Keeping your head online has some viability for someone who throws with a very low elbow slot.
And what keeping your head online is mostly about asymptomatic that most players don't know how to sequence the body i.e. a do not know how to create a kinetic sequence i.e. leg rotates around transferring to hips transferring to midsection transferring to upper body shoulders transferring to arm transferring to fingertips. What they try to do is lock up their entire body and then yank their head off in an attempt to spin around as opposed to creating a segmental sequence.
Your son is yanking his head around to some degree but I would work on getting proper rotation into foot plant i.e. not locking up the front knee so that it pushes the hips back and off to the side before I did anything else.
I like the way your son finishes he really is trying to throw the baseball. Please don't potentially screw him up by trying to make in the pitcher right now.