Back child support payments statute of limitations. Finding out after 18 years that your daughter is not yours.

My brother has been divorced for 15 years. He has 3 children and just found out that his last daughter is not his. She was 18 when they took the paternity test. His 2 other children are 23 and 21 and live with him. His ex is still wanting back child support payments for all 3 children.
Is he still obligated to pay child support for 3 children, one of which is not his?
Can he sue the ex or the real father for child support payments? - Is this your question? Add additional information