Every child needs a GOOD Mother. but dont feel bad that your son lost his mother's love. It doesnt seem like your wife can be a good and loving mother. It is better not to have a parent than have an abusive parent. You being so mature can ensure that your son gets all the L & A.
In olden days, a wife would toil and bear all sufferings for the sake of her children first and then her husband. These days ...
CUSTODY -> she may be applying for custody only to satisy her/her family's EGO and not really to care for the child. Worse, she may want to emotionally kill you or blackmail you by taking away your child. Her intentions dont seem good or balanced to me, If she was so concerned, she would try to keep her marriage intact.