Ex is suing me for child support he claimed was overpaid?

My son is 17. Last year, I sent him to live with his father out of state, due to drug use and the inability to lawfully get him into rehab. Despite my telling my ex that he would still owe back child support, but it would end up going back anyways to him (because of my new child support order), he still thought his child support arrears would be dropped, because my son was now living in his home. I just received notice from my county court office, that he wants for me to sign an enclosed document, saying that he satisfied his back child support (that has the current balance of $3500), but that he also wants to be paid back another $4,000, because he believes that his $300 a month court-ordered amount was set too high a few years ago, causing an overpayment. (Not only was he receiving unemployment, but he was also working under the table. Unfortunately, the latter cannot be proven, but with state unemployment, child support based their court order on that calculation.) He claims that he now needs this $7,500 to be applied to our son, so that he has money to take care of him until he turns 18. I think this is crazy. There were months, where he didn't even pay child support, and now, he just wants to get off the hook of owing me the back pay (since I had to do it all myself), and since he realizes that he can't declare bankruptcy on this. Should I be concerned? Or should I relax, because as I said, it seems crazy. After all, I'm not working, I'm married, have another child who lives with me and yet I manage to pay my court-ordered child support to my son on time. My son's father, has no other children, and I remember times where I was unable to pay for school pictures, and asked if he could spare $20, and he said he didn't have it. I would then find photos posted on social networks, where he and his girlfriend went on vacation to Bermuda. (I wish that I could bring these photos in as evidence, but unfortunately, I was told they would be irrelevant to the case.)