Ex has filed for child support to end as of last June and wants me to repay him from that date. Can I be held liable? Support is getting withheld from his pay. I thought it read to receive until this April. It seems was suppose to stop when son graduated which was in June or this April when he turned nineteen if he didn't graduate.

Yes, if a court deems he over paid, you could be liable for that overpayment.