Root- Great discussion. The issue: You're looking at Vertical break wrong. That positive number is the amount above a straight line from release to the end point. All Fastballs have some degree of backspin. So, your son's smaller positive vertical break means that he has more drop than the others (relatively) but still higher than a straight line. A curveball will show negative vertical break, because the spin makes it fall faster than a straight line.
Even changeups generally have a positive number but smaller than a fastballs positive number, because there is still backspin.
So, in short, higher positive numbers are rises over the straight line (it doesn't mean the ball went up, just that it's fighting gravity) and negative numbers are downward breaks below the line. So an 11 vertical break means it was up due to backspin, but not as much as 16 or 20' so relative to other fastballs it acted more like a sinker.
Here are the descriptions in Trackman (Rapsodo is similar):
Does that help?