If you and your son both believed that college was assured then this happening out of the blue must be a big worry..so I sense your concerns.
Yes plan for the future when funding must be sourced but assume that this year is ok..
maybe setting up a meeting then asking in front of a nuetral party..say a church member of status if your ex has an important role there..to state clearly this years intention and that the fees are due.The fact he is recovering means that this support could benefit your ex too.
Funding any degree is expensive..my daughter has three and my son two.They both worked during vacations.My own studies were paid for by myself.so I speak from life experience But the stress of study is sufficent without additional hassel.
I hope this is resolved soon