Could submitting to DNA test result in arrears?

My partner has just been told by an ex that he has a 15year old?. IV told him to get a dna done before anything happens,his ex is trying to get him do do in with the csa but he wants to go private?

She’s calling all the shots and says her way or no way? We know that she was looking for my partner for years but couldn’t remember his surname? She’s said at the beginning which has only been a week she’s not looking for anything off him.

If he does a DNA with csa can they hit him with 15 years arrears? She’s been asking about his job and question about him alot but we don’t know much about her situation. Pls help.