Agreeing and adding....
I agree with CADad that his front shoulder may be opening a hair too soon and, to allow for better assessment, that you should also video him from the open side and from behind. I also agree with Chris that his kick may be a bit too far out and that he is landing a bit more on his heel than on the flat of his glove-side foot.
A few other suggestions...
His glove (and pitch grip) is open to the first base side which could allow a base coach to see the pitch selection and be able to tip off his hitter. While coaching the bases, I try to use anything a pitcher will give me to help my hitters and an "open" glove has at times "helped" our offense.
Standing a little "taller" in the delivery (a little less collapse of the back leg) will permit a higher release point. A little less collapse may also eliminate the slight heel-first landing since it probably will shorten his total stride distance.
Also, a small issue that has created a little control trouble for some of our pitchers is the east/west movement in the step-back (as opposed to using a more diagonal step back). Since the diagonal step permits a more north/south transition (keeping the body more in line with a backward/forward movement to the plate from beginning to finish) it solved some minor control problems for us.
I have no medical background but I'll never forget what a doctor told me early in my coaching career when I inquired as to why one of my very BIG pitchers wasn't able to throw very hard. Though a 15 or 16 year old boy may be big on the outside, the muscles/bones were still growing/developing on the inside and sometimes were not "equal" to the outside dimensions. In short, he was still literally growing "into" his body and, as a result, experiencing some difficulty (i.e. velocity, control) while the growth "adjustments" were taking place. This may also be the case with your son.
Lastly, while I agree (as others have stated in several posts) that your son looks very good, I think recording his delivery on video and posting it on this web site was smart of you to do. I've always believed that if a coach's own education couldn't provide him with an answer, he should never hesitate to "use" the knowledge/experience of those around him. Both he and his players would be better for it. Fortunately, this web site has some very gifted people who are always willing to lend a hand when asked...and as a dad who wants the best for his son, you did the right thing.
Good luck and keep us posted as to how his season goes.