In MOST circumstances, attorney-client communications are (a) protected from disclosure in court by the attorney-client privilege, and are (b) protected by attorney's duty to keep confidential his client's secrets. Thus, in general, a confession to an attorney about a past crime committed will be kept confidential and cannot be used in court. There are exceptions--for example, you can't insulate EVIDENCE of the crime by bringing it to your attorneys' office. You can't go into your attorney's office and say "I killed X, I'm a serial killer, and I'm going to kill again; help me avoid detection." The attorney has a responsibility to report any future crime and, certainly, cannot assist or participate in any criminal activity. These state confidentiality and attorney-client privilege rules vary from state to state, so my suggestion is, before saying what he's done, talk to an attorney about the scope of the confidential communications.
cvq3842's comment is mostly right. If a client confesses to an attorney, what they can't do is allow a client to testify when they know they're going to perjure themselves. An attorney can still argue for the person's innocence or guilt (either because the attorney doesn't believe the confession or doesn't believe that the evidence proves guilt).
What kind of divorce are you talking about? You should be married to just one partner and he should be easy to find. If you are talking about child support that is a total different issue.