OP, thank you for all of your clarifications - I know it is not fun going through what you are enduring, and I hope your ex steps up. I think your story will be very helpful for a lot of families, especially as a reminder to those that even at the very generous schools like Amherst, it is important to try to estimate the cost over all 4 years as your family situation changes.
Our experience with the Amherst FA department was very different. They were not only very helpful when we met with them before admissions - there was a gentleman there who made it clear that each year’s FA application was for one year only, but he told us “without giving any financial advice, here are some of the questions you will probably be asking yourselves as you compare our program to the schools we compete against”. They get lots of questions from families with multiple siblings and they really do want to help. I recall being surprised when he told me that there were a lot of students whose families take out loans, even though they try hard to come up with an aid formula so that students don’t have to do so. But every student and family has different priorities, and while there are students who want to work during the summer, there are also students who will prefer to travel or do research or lots of other things. Amherst generally will exclude a significant portion of home equity on the principal residence, and they don’t expect parents to stop contributing to a retirement plan during a child’s college years. Some schools do.
It is clear that you agree that Amherst is very generous, and you agree with them that based on what you know of his father’s financial situation, this is not a concern of whether or not he can afford to, but instead whether he will agree to do so. I fully understand that due to its price tag, Amherst may not be seen as value to many full-pay families, but there are many families who happily pay even though, lets face it, a student accepted to Amherst may have merit options at other schools.
I think your approach sounds more than fair, with you and your ex both doubling your contribution, and you son taking a Stafford loan. Will you need to take a PLUS in order to double your contribution from $5K to $10K? Do you think your ex will kick in another $20K over the last two years? You mentioned earlier he has been very unwilling, especially if you ask, so is your son asking him? Have you told your son that if his father does not increase his share that you don’t know how to make it work?
I can understand a child being reluctant to asking a NCP for additional money for an expensive school. I would recommend that if he does ask, he explain that he himself is getting a loan, and (if so) he intends to pay both mom and dad back in full. In order to appease his father more, perhaps he agrees to pay his dad back first before he pays anything back to mom. Your son is a young adult and is learning about how to solve some of life’s problems - would it be better if he asked his dad if his dad knew of any other possibilities besides dropping out/transferring? When S was asking us for financial assistance on his new home purchase, he told us afterward he realized the best way to ask us was to show us that he’d thought of multiple possibilities. But by now, your son probably knows the best way to reach out to both of his parents.