Maybe, maybe not. The Fraud wasn't that he agreed to sign on as the parent, the fraud occured when she declared him as parent in a support order.
Legally he could take her to small claims and sue her for illegally collected support especially if it was paid on a wage order and he had no way to refuse.
I agree this probably wouldn't fly in family court, but it might be winable in a civil court.
To me it would be worth the $45 filing fee to find out. He could get an order for her to repay every nickle of support he's paid over the last 8 years.
The kicker comes where he is named as a parent in a legally binding document (the support) legally she couldn't petition for an step-parent adoption without him signing a waver of parental rights.
His rights would have to be terminated which would reguire a guardian adlitem for the child at the mom's expense and then she would have to prove that termination would be in the best interest of the child which in his case doesn't look easy to do.