@Base48, in selection processes (e.g., job search, college apps, etc.), it is easy to assess blame on “the other.” By way of example, many people blame athletes as the reason that they were not accepted to a given college. Sometimes it ends up just being the way the chips fall.
Your son applied knowing that there was no commitment on merit aid but with an assurance of admission. He received exactly what you thought, although later you learned that others with similar stats received merit aid. Comparisons are natural.
You don’t mention the type of merit aid involved. Some schools offer merit aid to everyone who has above a cut-off GPA and board score. Some merit aid is more exclusive, looking more for the whole package, and students may actually have to apply for it.
Your son is not entitled to merit aid, but he is entitled to an explanation of why none was offered. I would ask the people responsible for the decision for an explanation - and sure, tell them what merit aid was offered by other schools. Given the need for schools to compete for students (see Ivy League antitrust litigation) I see nothing wrong with showing what your cost of attendance would be elsewhere.
A final note. The coach really needs to stay out of this one. The NCAA Division III Rules state: “Members of the athletics staff of a member institution shall not be permitted to arrange or modify the financial aid package (as assembled by the financial aid officer or financial aid committee) and are prohibited from serving as members of member institutions’ financial aid committees and from being involved in any manner in the review of the institutional financial assistance to be awarded to a student-athlete.” Rule 15.3.4.