Most good private colleges are need based

you should apply for the financial assistance.

Since your son is 18, you should let him know the truth and let him help make a decision that is appropriate with your situation.

My guess is your ex probably wanted him to go to public school, but you think the private school has higher ranking. You are obviously a very loving mother. Since it's about your son, I think he deserve to know the truth and decide his own future and school. You need to evaluate all the possible pros and cons, and make a right decision.

There're too many irrisponsible fathers out there, since you can not rely on them, just have to forget expecting from them, and think of a way yourself.

Be strong!