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 tutoring.. 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..