Liz, what is your court ordered agreement about child support? I know in the state which I live it's over at age 18 unless they are in school, then it becomes 21. I also have in my decree that child cannot change schools w/out the agreement of BOTH parents.
I hate to say, if your decree says it's over at age 18, I would not think there is much you can do about it? If it does not, can you not simply wait until he does not pay his obligation then file for contempt? It's my understanding you can do that ProSe pretty easily when it's for payment issues?
Just a couple of thoughts.....
Hugs. Many hugs.