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First look at the existing court order for support. If it says he pays 1/2 the medical expenses, then he owes you 1/2 the medical expenses. Your Ex does not get to stop paying you child support because the son goes over to stay at his house for a few days or weeks. Any changes in child support have to be done by a court order. So, you win all around here.
All the best,
ANd if this is what happened, i.e., you did notify him each time before incurring the expense - then he owes you!