You are right, homosexual relations and acts are not part of God’s plan. We cannot affirm what God does not affirm. Homosexual attraction is a disorder, a defect in sexual orientation. If it is innate, the person is not to be blamed. They need to be loved and supported with compassion and with the truth.
So I encourage you to keep praying and making reparation for your son, that the Lord may grant the necessary graces to free him from the falsehood of the homosexual lifestyle.