Please tell him, being hurt is not an excuse for giving up on your wife.  She is his covenantal wife.  This means she and he are in a covenant before God that is not easily broken.

If I was advising your son, I would share with him that marriage has a bigger scope than just being happy.  Marriage between one man and one woman is to bring God glory. Mark 10:4-9  ⁴ And they said, "Moses permitted a man to write a certificate of divorce and send her away."  ⁵ But Jesus said to them, "Because of your hardness of heart he wrote you this commandment.  ⁶ "But from the beginning of creation, God made them male and female.  ⁷ "For this cause a man shall leave his father and mother,  ⁸ and the two shall become one flesh; consequently they are no longer two, but one flesh.  ⁹ "What therefore God has joined together, let no man separate."

Within mankind, there has always been the desire to hurt someone when you have been hurt.  However, a mature believer in Jesus Christ will seek restoration instead of separation within the relationship.  Someone may advise him that it is his right to dissolve the marriage because of passages like Matthew 19:9  ⁹ "And I say to you, whoever divorces his wife, except for immorality, and marries another woman commits adultery." Yes, he may have the right but that would not be God's choice.  God has already forgiven your son for things he has done and He will continue to do so.  We are more like God in our character when we forgive. 

Please share with your son that it is not over unless he says it is over.  God does not want it to be over.  So let me share with you the steps to restoration.  First, confront the offender, then forgive.  So he needs to confront the situation with the idea that God wants this relationship to stay together and get stronger.  He must understand that if she confesses the sin, then it is his responsbility to forgive.  Forgiveness is a choice.  To forgive is a promise he will give to her that he has removed all of the blame, and that he will not seek revenge or retaliation.  Luke 17:3-5   ³ "Be on your guard! If your brother sins, rebuke him; and if he repents, forgive him.  ⁴ "And if he sins against you seven times a day, and returns to you seven times, saying, 'I repent,' forgive him."  ⁵ ¶ And the apostles said to the Lord, "Increase our faith!" Matthew 6:11-15  ¹¹ 'Give us this day our daily bread.  ¹² 'And forgive us our debts, as we also have forgiven our debtors.  ¹³ 'And do not lead us into temptation, but deliver us from evil. For Thine is the kingdom, and the power, and the glory, forever. Amen.'  ¹⁴ "For if you forgive men for their transgressions, your heavenly Father will also forgive you.  ¹⁵ "But if you do not forgive men, then your Father will not forgive your transgressions. 

He must understand that no one wakes up and says that they are going to have an affair today.  There had to have been a break down in the relationship long before the affair.  Ask your son if he is willing to get counseling.  I know where he can get biblical counseling and it will not cost him one dime. 

2. Pastor, is it okay for me to maintain a friendship with another male that I once had a sexual relationship with and my husband doesn't know anything about it? He is a good friend and I want to stay close to him as a friend only.

It is not okay for you to maintain a friendship with any past lover.  You will always find some imperfection in your present mate just by comparison.  Our imagination will always find perfect in our past involvements while we face imperfections in our present reality.  When the people came out of Egypt they always wanted to go back as they faced hard reality of there present wilderness journey.  All they chose to remember was the meats and watermelon.  And what they chose to forget was all the beatings of their hard taskmaster.  You will never put the energy into your present relationship when you are always looking back.  This person is deep into your life and the further you get away from them, the more freedom you will have to love the imperfect gift God has given to you in your husband.  1 Corinthians 6:15-20  ¹⁵ Do you not know that your bodies are members of Christ? Shall I then take away the members of Christ and make them members of a harlot? May it never be!  ¹⁶ Or do you not know that the one who joins himself to a harlot is one body with her? For He says, "The two will become one flesh."  ¹⁷ But the one who joins himself to the Lord is one spirit with Him.  ¹⁸ Flee immorality. Every other sin that a man commits is outside the body, but the immoral man sins against his own body.  ¹⁹ Or do you not know that your body is a temple of the Holy Spirit who is in you, whom you have from God, and that you are not your own?  ²⁰ For you have been bought with a price: therefore glorify God in your body.

You may be still having an affair with this person in your mind and not know it.  The bible says we must flee immorality.   Matthew 5:27-28  ²⁷ ¶ "You have heard that it was said, 'You shall not commit adultery';  ²⁸ but I say to you, that everyone who looks on a woman (or Man) to lust for her has committed adultery with her already in his heart. 

If this is the case, ask God to forgive you 1 John 1:9 and He will because that sin has already been paid for by the death of Jesus Christ.  Go and sin no more.  God wants you to be madly in love with your mate as He is in love with the other members of the Trinity.  God wants you to spiritually grow and be just like him.  Matthew 5:48  ⁴⁸ "Therefore you are to be perfect, as your heavenly Father is perfect.  1 Peter 1:14-16  ¹⁴ As obedient children, do not be conformed to the former lusts which were yours in your ignorance,  ¹⁵ but like the Holy One who called you, be holy yourselves also in all your behavior;  ¹⁶ because it is written, "You shall be holy, for I am holy."

The Bible is written by God.  Wedding vows are of course written by men and are influenced by their interpretation of what they think God meant by what He said in the Bible.  Wedding vows with the word “obey” in them, may be a misintepretation of the Biblical principal, “be in subjection” or “subject your self under your husband”.  The word hupotasso does not mean obey, it means "rank yourself under".  The word is a military word which means to line up under.

 ^NAS Colossians 3:18 ¶ Wives, be subject (Hupotasso) to your husbands, as is fitting in the Lord.

 ^NAS Colossians 3:20 Children, be obedient (Hupakouete) to your parents in all things, for this is well-pleasing to the Lord.

 ^NAS Colossians 3:22 Slaves, in all things obey (Hupakouete) those who are your masters on earth, not with external service, as those who merely please men, but with sincerity of heart, fearing the Lord.

 When it comes to wedding vows, the wife is told to be subject to your own husband.  When it comes to slaves and Children, the word means to to obey for example  "obey your father and mother, and slave obey your master."