my husband has a son previous to our marriage

[mommaof3]

My husband and I have recently found out that he has a son previous to our marriage by listening to rumors and finally finding the mothers mom and talking to her. The mother and her husband(the man on the birth certificate) are getting a divorce now. But I was wondering if anyone knew if it is illegal to sign a birth certificate if you know you aren't the dad, that's what he did? And then on top of that, to hid the boy and deny him any relationship with his real father. Can we force a DNA test? I have email from us to the mothers mom that I think we could use in court but I don't know. I need advice