Salam Alaykum everyone!! I hope everyone is safe and doing good Inshallah!!

So to get straight to my issue. I am married with 2 kids, ages 17 and 9.. My husband and I teach both kids Quran and they also have a Arabic and Islamic tutor.. My 17 year old son is born here but very familiar with our cultural and understands our religion.. Well I found out he has a girlfriend for a little less than a year, I was shocked to know that he slept with her many times, and they have a very inappropriate relationship. She is Atheist my son wants to marry her.. My life has flipped upside down and I am just completely shocked.. plz give me advice. I really appreciate it..

Edit: Thank you very much ladies.. I have been thinking about this situation day and night and talked to my son alot more.. I even talked to the girl, she has told me that she knew what they were doing was wrong, and that she loves him, I asked her alot about her self to get to know her.. She seems like a good person trying to figure her self out.. My son told me that she will convert, but I don't want her to convert for him only, I want for her to do it if she wants but I left it in God's hands and prayed for the best but today they decided mutually without my interference that it will not work out because they realized they do not have the same goals or values.. I actually feel very hurt because he is heartbroken.. I hope everything works out for the best inshallah.. Once again thank you ladies for your replies..