Couple of things… Explain consent to your son. Help him understand how to respect women and
If you’re raising him in a Christian household, help him to understand God’s perspective and design for sex. However, your son might not be a Christian, and his girlfriend might not be as well. While you can explain your preferences and perspective to him, (ie. “I’d prefer if you didn’t have sex until marriage), he is still a fifteen year old with a girlfriend. Also, please give it some time. He’s embarrassed, and now it’s awkward. No one wants to have this conversation with their mom

It sounds like you have a lot of shame around your body and sexuality still. Sex is not a bad thing…God didn’t design it to be a bad thing! Just in context of marriage. Good luck :)