Lost, yes. Forever lost? Not unless he chooses to be lost forever. Have you never read the story of the Prodigal Son (Luke 15:11–32)? That young fool left home but eventually came to his senses and returned to the father he had treated so vilely. His father was waiting for him, which is the point of the story. Repentant prodigals are a dime a dozen. A waiting father—in this case the heavenly Father—is a miracle of grace.
The apostle Paul is scornful of perverted teachers who “destroy[ed] the faith of some” (2 Tim. 2:18). Nevertheless, Paul immediately affirms, “The Lord knows those who are his.” He knows them, even if they do not know that they are His. In my judgment, this passage seems to indicate that it is possible for one to lose his faith, yet not that vital connection with the Lord that, ultimately, is the Holy Spirit’s to maintain.
The chapter ends with the possibility that some who, like the Prodigal Son had turned aside from the paths of life, “will come to their senses and escape the trap of the devil.” You can pray in the assurance that God will do everything divinely possible (in keeping with His character) to rescue your son.