Putting aside the ethics of backing out of an ED admit, I wouldn’t put aside a sure-thing acceptance based on the verbal statements of a coach made in December of senior year.
Coaches do not make the final decisions about admissions or financial aid. Coaches are also known to leave schools after recruiting athletes and the athletes’ playing time and/or scholarships disappear. And it is very late in the game for recruitment, which suggests your son was not a first choice/top recruit and will be treated accordingly.
It is flattering to have a coach want you, but I don’t think a student should attend a school they would not consider if not for the athletics. They are always one injury away from losing a spot on the team and athletic scholarships; would they want to be at that school then?
So I think there are both ethical reasons and practical reasons to not jump ship.