Oh lord its a challenge.
Unless you spk to the girls mum you'll never know whether his girlfriends little sister is really having her boyfriend over or that's a story to make you feel more relaxed, which has backfired on him.
The best way to manage this would be to ask for mothers contact details, not from your son, but his gf when she next come over.
Conversations about legality of having underage sex, what is she changes her mind and reports him further down the line school, which does happen and then school/police ibecome nvolved.
Remind him you both love and trust him to be sensible, unfortunately, there isn't much you can do tbh as he's lovestruck and in the throes of first love, so not thinking clearly.
Have her over but set some ground rules. Homework, friends, after school activities etc must continue. he must be respectful of family/younger siblings etc
I would spk to his gfs mum, say happy for gf to sleep over but your son wbel be respectful and sleep on sofa, she'll get the hint and hopefully will be thinking the sam
e and respond in kind.
Its a tough one but if you go nose to nose with him, it will be tough.
Remember, he will come to his senses