Ugh! I'm so sorry you are going through this with your mother! She sounds so insensitive. I agree with some of what others have said- just try as well as you can to cut her out of the process.
On the issue of financial aid- the FAFSA is the actual application, not any sort of aid itself. The aid varies tremendously from one school to the next (Pell Grants, State Grants, Loan programs, etc...). As far as I have ever known, you *always* are asked for your parent's tax returns if you are under 24. Now, there *is* a way to file some sort of exception to be declared an "independent" student. Maybe that is what your other kids did before?
Financial aid should be able to tell your son how to do that. Being asked for tax returns is the norm though. Your application will sometimes be selected for verification as well, in which case they will ask for additional information. That has happened to me *EVERY* year I have been in school and has involved having to supply *TONS* of information including tax returns, lease, bank statements, etc.... Some of this is just the nature of financial aid unfortunately, but your SON should be the one doing all the leg work. It is a shame your mother got involved.
As a side note- things may be easier at a public school, not a school like DeVry. My brother went to DeVry and his degree wasn't worth the paper it was written on and to add insult to injury, the degree cost him *THOUSANDS* of dollars. Your son's mileage may vary.