How can I help my 22-year-old son who doesn't pray, excessively drinks alcohol, and is involved with an unsuitable girl, and who regrets his actions but cannot free himself from them?

Do not despair of Allah's mercy, for despair and haste prevent the prayer from being answered. Among the most powerful factors influencing behavior are choosing righteous companionship and leaving a bad environment, as stated in the Qur'an and Sunnah. Therefore, advise your son to choose righteous company that will keep him away from forbidden acts such as neglecting prayer, drinking alcohol, and illicit relationships. If possible, travel with him to a place where righteous people reside. Strive to pray for him, especially during times when prayers are answered. If you find a righteous girl for him, one of good character and religion, who accepts him as a husband, that may help straighten his behavior and maintain his chastity, provided you do not deceive her about qualities he does not possess, as she has the right to inquire and ask about his situation.