All you can do is keep working with Cornell FA and because of the late date, I would elevate your concerns to a higher level admin person.
Have they told you how they calculated the most recent FA offer? And is that’s what is in dispute, or do you still not know why/how the FA packages changed? This really has gone on such a long time, I feel like we are missing part of the story (again that’s ok, you need not share everything!)
Make sure your S deposits at another school by May 1 if Cornell details aren’t worked out by then, he doesn’t want to lose his other acceptances.
Also…if you don’t have clarity with Cornell by May 1, I would formally ask them to extend past the May 1 enrollment deadline, and get that in writing (but your S still needs to deposit elsewhere.)