I know we are getting into the weeds now, but as @thumper said, FAFSA changes all the time. If you filled out a form for practice, just to see what your number would be for Pell grants, I think the rules for you will have changed.
" For the 2026–2027 academic year, the parent who files the FAFSA for a student with divorced or separated parents is the parent who provided the greater portion of the student’s financial support during the 12 months prior to filling out the application.
This rule applies if the parents are divorced, separated, or never married and live in separate households. The FAFSA no longer uses the “custodial parent” (the one the student lived with most) as the primary determinant, having shifted to a financial support-based test for the 2024-25 and subsequent, including 2026-27, cycles"
If you make $70k per year, and get $3500/mo ($42k/yr) in child support, it’s likely it would be determined that their father is the parent who provides the most in support. Some of your $70k is to support YOU, so you’d have to figure out how much is supporting children since you’d have deduct your taxes, insurance, car, food, etc. from the gross income.
I think most families would use the worksheets from the state child support calculations, but you said you don’t use those worksheets. You may have to fill one out just to see what portion would be attributed to you and what portion to him.
You also keep talking about how much money your son earns and how he could just work more while in college. That will also reduce the federal aid (Pell, SEOG, subsidized loans) or for schools that only give need based aid, reduce aid. That’s why many on College Confidential recommend taking the full merit awards from NMF or a named award like Stamps. Going thru the FA process every year is not fun, especially if income and costs change every year and you are worried about having to drop out because you can’t afford it.
If he gets a full ride at several schools, THEN he can be picky and decide he prefers Harvard over Alabama, but until he knows that the costs are do-able at the school in the cold climate with skiing nearby or that has the girls he’d like to date, cost HAS to be considered or you are just wasting your time. My kids could have gotten into a number of schools in California (their preference) but I couldn’t afford them so I wouldn’t let them apply until they could show me how they were going to pay for that school. Like you, I was a single mother sending two kids to college in the same year. Unlike you, I wasn’t getting child support. We had to make the money work first.