One of my kids divorced his wife and remarried. How do I put his new wife on the family tree

Go to your son's profile, and then down in the Spouses and Children section, click the Add Spouse button. You can add as many spouses as you'd like to a person. You probably will want to update his relationship to his previous wife and add a Divorce event. You may also want to change the new wife to be the Preferred spouse; to do this, click Set Preferred in the Family Members section of your son's profile page.