You can't save that person. You can't stop him from pursuing the gay life. You must learn to trust God to draw your loved one by His Spirit. Trust Him to protect your loved one. God's desire is to set him free from sin and deception.
Prayer combined with fasting is a powerful spiritual weapon.
Jesus knew prayer and fasting got results. He practice it, as did His disciples. Throughout the Bible, when men and women faced difficult circumstances, they prayed and fasted. God often responded in miraculous ways. Though God does answer our prayers, He rarely answers them when or how we want.