If my name is not on the birth certificate , am i still subject to paying back child support?

[Kristina]

My husbands ex put another mans name on his sons birth certificate, and even though he was exempt from having to pay any CS when we did a DNA test and took her to court to have paternity re- established, we were told because of her threats that after a child support case is open retroactive child support is gone. She has to ask for it at the case hearing, she can't do it later down the road.