J.W.
Pretty sure you can sue him in small claims. Just make sure it is a medical expense as defined by the IRS.
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Is this about the counseling you mentioned in 2011? Counseling is not considered a medical expense according to the IRS and that is how family court determines medical expense. The only way he would be on the hook for his 80% is if he agreed to pay for the counseling before the expense was incurred.
This one I know quite well which is why I said in my original answer if it is considered a medical expense.