I think that we should try not to judge the OP’s son’s intentions and always pray for the best.
Having been a 21 year old man who lived an immoral lifestyle and regularly blasphemed our Lord out of rage (

), I can say that when your son does truly turn back to God, you can expect it to be something that rocks the very foundations that his life is built on (it will shake and disturbs his very being).
The reason for this is simply that Satan fights like crazy to stay in control. When I first felt my conversion to God, I was living comfortably in a sinful relationship. I started to feel this love for God that I had never felt before in my entire life, but to embrace it in any meaningful way would require me to go through a break-up. Out of fear of losing the relationship I was in and having my life seriously inconvenienced, I initially decided to hide from God’s love. That warm love that I was finally feeling didn’t go away but rather turned to an aching anxiety in my chest that burned for about a month and a half. I suffered from extreme anxiety and an inability to sleep at night all the while fighting to maintain the sinful relationship out of fear of a breakup. During this time I became physically ill and it was really the closest thing I could describe as being hell. As a result of my altered behaviour (and, of course, God’s graces) the sinful relationship finally ended against my own will in late August. At that point in time I decided to embrace God’s love, the pain in my chest instantly turned into warmth again and I felt no sadness over the break-up, whatsoever. Since then I have made God the focus of my life and was confirmed in the church this past Easter.
I will certainly pray for your son, since I know exactly what he is going through. Don’t worry, because through God all things are possible. I truly believe that I was led back to the faith because of the many prayers that my grandma had said for me. Just remember to pray for him each and every day!
God bless!