It sounds to me like you have a complicated support issue that involves two different states. If I were you I would talk to a experienced family law attorney in the state where you live and get their opinion on what will happen if your son pushes this further.
If the medical bills are less than what it would cost to obtain a lawyer and fight it, you might want to consider paying the amount.
Unfortunately, I agree with the first lawyer that thinks this requires the input of a lawyer in Indiana with expertise in family law issues.
The typical answer to your question is that an adult parent is not responsible for the medical expenses of their adult child. Thus, the only way you may owe this money is based upon the terms of the child support order.
It is for that reason that the answer to the question likely does not lie in "the law" in general, but in interpreting your case, in particular. I know it is better if you can get some more guidance on a question like this, but the reality is that the law does not require you to pay, it will take a meeting with a lawyer who can help you look at the case and intrepet it based upon other rulings in that state for you to be able to determine what, if anything, you owe from these unpaid medical bils.
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