Without more detail, it's hard to determine whether he is legally responsible for the debts. A bankruptcy will get rid of them so he should probably consult a local experienced bankruptcy attorney ASAP. It could be that he won't need to file bankruptcy but only a consultation with an experienced lawyer will answer that.
The answers to these questions may be different depending on your individual circumstance and should not be considered as legal advice or the establishment of an attorney-client relationship.
He needs to consult with a bankruptcy attorney. He can do a search on AVVO or he is free to email me for a referral as I and many others on here know some reputable bankruptcy attorneys that charge reasonable fees. Good luck to your son.
Bankruptcy would seem to be the only practical answer. His ex-wife may have some liability to him if he was defrauded during the marriage, but he would probably have all sorts of proof hurdles and bankruptcy hurdles and come up with money he doesn't have for lawyers to fight through that mess all to get a judgement against a woman who doesn't have any money to pay it. I don't know if what ex-wife did rose to the level of a crime but it might merit some thought.... still though it sounds as if bankruptcy is his only option.
My best advice to you is to seek the advise of a competent bankruptcy lawyer. Most bankruptcy lawyers offer a free consultation. Make sure that you select an attorney that has been practicing law for a considerable period of time and who's emphasis is on bankruptcy law. When you call to make an appointment be sure to find out what documents you should bring to your meeting in order to get the maximum benefit from your free consultation.