How can a committed young man — who had abandoned masturbation and pornography — avoid relapse and a return to them, and regain his vigor in obedience and worship?

Life is a realm of trial and test, and in it, a person is susceptible to periods of languor after being active in worship. This languor may extend to abandoning recommended acts or falling into disliked ones. If this languor reaches the point of abandoning obligatory duties and committing forbidden acts, then this is a serious matter, and one fears a bad end for the person involved. What is required is sincerity with Allah, firm resolve, and frequent supplication for Allah to keep the heart steadfast on His religion.