My question involves child support in the State of: Texas

I have just become engaged to a father of one. He and his ex were never married but they did have one child. The mother never filed for CS and custody has never been set. The parents have made their own arrangements. He pays her medical bills (she is on his insurance) and provides clothing and any assistance she needs and has for the past 13 years. Our concern is that when the ex discovers he is getting married, she will file for CS.
My question is, should this happen, would the courts likely go back for the full 13 years even though he has been paying under their own arrangements? It scares me to think of the financial strain it could potentially put us under and am just looking for a little insight as this is all new to me.