When BD owed over 5 grand in back medical expenses, we negotiated for him to pay the full cost of braces and called the extra $200 he owed a wash. That worked out very well for us. We had to have the help of a mediator to do this; there was not agreeing to anything unless it was what he wanted. What he wanted was not always in DS's best interests.
Try mediation, if it's available. We just wrote this into a motion shifting custody to BD when DS was 14.