Your son knows better than I do why he turned away from God although many young people follow the same path. Thankfully, for many it's only a temporary detour, and as they grow older and see how serious and complicated life is, they realize they need God after all.
But whatever your son's reasons, the ultimate problem with most people is that they want to run their own lives instead of allowing God to run them. To them, it sounds attractive to be independent and in control, because then you can do anything you want. Jesus warned that sometimes God's Word takes root and begins to grow, "but the worries of this life, the deceitfulness of wealth and the desires for other things come in and choke the word, making it unfruitful" (Mark 4:19).
And you can be sure that Satan will do everything he can to reinforce this, whispering into our ears that nothing could be more boring or restrictive than following Christ. But it isn't true! The Bible warns, "There is a way that seems right to a man, but in the end it leads to death" (Proverbs 16:25).
Pray for your son every day, and ask God to convict him of his sin and help him realize the emptiness of his life apart from Christ. God loves him, and someday the seeds that were planted in his heart years ago will begin to bear fruit.