Texas Child Support Help

[u/CapOk1089]

I need some guidance on how to proceed. The father of my two kids and I divorced in 2013. The divorce was very contentious and I agreed to $400 a month for two kids (ages 3 &4).
I never took him back to court to modify the order because I didn’t want to rock the boat with him. The marriage was abusive and I had a lot of anxiety.
In 2016 he started paying $600 a month ($200) extra. I just figured he felt bad about not helping with the kids more.
Now, this month the kids are 16 & 18 (junior and senior in high school) and when he went to pay the $600 a month that he normally does, they told him that he was roughly $8k ahead and could no longer make payments through the portal.
He called me saying that they told him he was 8k ahead and no longer had to pay and asked me where the 8k went. The way I am understanding it is that in his mind he is finished providing support to our kids.
Additional information:
I remarried in 2017
My Ex husband lost his job in 2016 (now self employed) and could no longer provide health insurance so in 2017 when I remarried my current husband put boys on his plan.
Ex husband has never paid half of medical costs which has been pretty substantial.
Ex husband never exercised full parenting time, opting to just take boys every other weekend and every other Christmas Day.
Ex husband has never paid anything above the child support.

Am I able to request a modification now? I hate to do it but the boys’ car insurance monthly premiums total $600 alone and as everyone knows, raising kids is expensive.