You don't have all the facts.
Was the woman married to the other man when the child was conceived?
Did the other man file an AOP and later marry the woman?
When did they get a divorce, did he question paternity at that time?
Be that as it may your husband will still have to respond to the paternity action. Is this simply a letter form an attorney or has a paternity action been filed?
Did he know about the woman's pregnancy and was he having sex with her at about the time the child was conceived?
If he is proved to be the bio dad of this child and mom never received state aid, then your husband should only be responsible for child support from the time of filing the paternity action and he would also be entitled to file for custody and visitation. How a judge will handle the situation will depend on the facts. The State of Tennessee uses an “Income Shares” model for determining child support payments. Income Shares requires the courts to review both parents’ incomes before setting a child support amount. Here is a link that may help.
http://tennessee.gov/humanserv/is/isfaqPrev.htm Whether or not DNA was available in 1990 may also be a consideration so it would be wise to consult an attorney.