We are truly sorry to hear about the struggle your son is going through. It's important to remember that God does not want us to live in guilt and condemnation, for it is written in Romans 8:1, There is therefore now no condemnation to those who are in Christ Jesus.
Your son may feel responsible, but God wants him to find freedom and forgiveness. It seems there may also be an element of not forgiving himself. It is not God's will for him to punish himself. It is good to have a contrite heart about what happened. However, he needs to know that he has forgiveness from God. First, remind him of the truth in 1 John 1:9, If we confess our sins, he is faithful and righteous to forgive us the sins, and to cleanse us from all unrighteousness.
Please know that it is only by the name of Jesus that we have access to the Father. Let's pray together for your son:
Dear Heavenly Father,
We come before you today, lifting up our brother who is carrying a heavy burden of guilt. Lord, we ask that you help him to understand that this situation is not outside of your sovereignty of control. Father, we know that you work all things together for good for those that love you and are called according to your purpose. Father, we pray for his healing and for him to have a right and balanced view of what happened. Please help him to see that you are not mad at him and have not left him. Father, help him to know that you want to have a relationship with him. We pray that you would restore his joy and peace, and that you would help him to be the father and husband you have called him to be. Father, we pray for his family and that you would restore what the enemy has sought to steal, kill, and destroy.
Father, we pray that you would strengthen him, his marriage, and his family. Father, let the love of God be shed abroad in their hearts. In the name of Jesus Christ we pray, Amen.
In addition to prayer, encourage your son to seek godly counsel and support from his local church community. He needs to be open and honest with his family about what he's going through. But more importantly, he needs to know the truth of the gospel.