[CT homeschool law](http://www.sde.ct.gov/sde/lib/sde/pdf/curriculum/homeschool/home_schooling_information_packet_2013_14.pdf) requires that parents file paperwork with the district. That can’t be done retroactively.
That’s not always true.[Some CT community colleges](https://www.manchestercc.edu/enrollment/) apparently require [proof of high school graduation.](CSCU - Admission) NYS colleges do too.
The [url=http://www.xaverian.org/page.cfm?p=370]tuition[/url] at Xaverian is ~$13k this year, so I’m not sure how the amount owed can be $20k. I didn’t think schools would let you enroll if you owed a balance from the previous year, or let you return for a 2nd semester if you hadn’t finished paying for the 1st.
Does your son have high enough stats to qualify for merit aid? Can you arrange a payment plan with the school? What’s your ex saying about the bill and your son’s plans for college?