If your son’s marriage was valid on his wedding day, nothing that happened later in the marriage—not even adultery—nullified it. That said, adultery may be evidence that your son’s wife did not enter into the marriage with the proper commitment required for a valid marriage to come into existence. If such is the case, an annulment may be possible.
On the other hand, if your son’s marriage is determined to be valid, the reality of the situation may be that he is married to a woman who is, tragically, not a good wife. Separation and civil divorce can protect him, his children, and his assets, but it cannot free him to remarry.
Remarriage in such a case will only be possible after the death of his wife or, if your son’s marriage is not sacramental (i.e., one or both spouses are not baptized), it might be possible to have the marriage dissolved.