just an idea, some say that it won't work, but it did and still does for me.
you don't give alot of details from your post, but i will assume that he is responsible for a % of the costs for medical after insurance, and that he isn't paying his part?
my ex did the same for over a year. which included about 15 colds and flu, 1 tonselectomy, 2 sets of ear tubes, and 1 hospitalization for a week. a pretty good lump sum even after insurance.
he paid his support - no choice, had that garnished - but he said that included his share of medical bills. WRONG!!!! If that were true, there would be the added line that states - NCP will carry medical insurance on the children and be responsible for 50% of uncovered costs.
we were running expensive bills on attorneys just for the regular stuff - my laywer suggested that i do it myself through small claims court. i had already been awarded the amount through the regular support order. he was responsible for it.
i took a copy of my order, along with copies of all the medical expenses to the court house with me. had it drawn out what the total was, what i had already paid - with receipts - and what he owed. i won! the only choice that he was given was rather it was to be paid directly to the doctors office or to me. he chose the doctor's office. the judge put a provision on the decision that stated if the doctors were not paid within 30 days that the amount would fall due to me - that enabled me to file another garnishment on his check and allowed me to pay the doctors.
the first time i did it was in 1997 and the second time was in 1998, third in 99. he has done alright since then. i present him with a copy of the billing, along with the amount that i have paid and receipts showing i paid my share. if he doesn't pay it within 2 months or at least set up a payment plan with the doctors, i send him a letter requesting that he pay the amount in full or that i will again take him back to court for the amount. with a reminder of his obligation and how important the welfare of the children are. he has done pretty good so far.