Am I obligated to continue my engagement to a young man who does not observe his prayers and fasting and deals in usury, despite his desire to change with my help, and would marrying him be a sin given my knowledge that "whoever abandons prayer is a disbeliever"?

One who abandons prayer is an unbeliever, whether out of denial or laziness. It is not permissible to marry one who abandons prayer unless he repents and shows signs of righteousness and uprightness. If the marriage contract has already taken place between you, it is invalid, because a Muslim woman is not lawful for an unbeliever. A new contract must be concluded after his repentance.