My son is 14 years and suddenly he does not believe in the existence of Christ.
There really is no case against the existence of Jesus, and so there is not much to say in the face of such a claim than to point out that it is the person making a claim who has the responsibility to back it up. The person denying the existence of Jesus must deny the validity of all of the historical data that we have about him. This includes the scriptures themselves, but also the extra-biblical evidence. We see mentions of Jesus in the writings of Tacitus, Suetonius, Pliny the Younger, Josephus, and others. We also have physical evidence such as the ossuary of James. That ossuary contains an inscription identifying the James interred within as being the James who was Jesus’s brother.
Below is a link to a debate between Richard Carrier, who is famous for his denial that Jesus existed, and Dr. William Lane Craig. Perhaps that debate will be helpful for your son?