Child support

Okay, so my husband found out that he has a son from a one night stand. They did dna that proved he was the dad. So she’s taking him for child support. I was wondering if he’ll have to pay back child support because he never knew that he was the dad. She had thought a different guy was the father. And since we’re married will they take into consideration that he has another kid with me and two step children when calculating child support? I don’t know how all this works.