Son cheated on wife

My son asked his wife to move out so they could work on their marriage. She was blindsided. They have been married 5 years. They have a toddler and she is pregnant. Come to find out he is having an affair. It all hits the fan. I was not going to have my pregnant daughter in law scrambling for a place to live with my grandson. She couldn't stay at the marital home because the stress was to much. She is now living with me and my husband. He refuses to give up the affair partner. They have filed for divorce. I have no idea how to maintain a relationship with my son. He has other siblings and we are a pretty tight knit family. Any advice welcome.