Your ex can't take a loan out and expect you to pay a part of it without either a court order or your agreement.

Second, contact a family law attorney in your area. S/he would be more familiar with how child support is calculated in your state. If it takes parenting time into consideration, file for a modificaton based on the 50/50 parenting and the one child graduating high school (support will continue until then).

Let him take you to court for the college expenses. You could probably get a judge to order that the payments go directly to the 18 year old as long as the 18 year old isn't living with your ex. That is, if they order child support to continue.