15 year old stepson possibly having sex without communication

I have a 15 year old stepson who has been in a relationship with a girl for about 4 months now. He has had a few minor relationships in the past but they have only lasted a few weeks. We knew this relationship would be different because they seem to vibe better than some of the others. We pulled him aside and gave him the "talk" of we prefer that he did not have sex right now but if he did, please let us know so we can get him protection. Or if they even just started fooling around in general. The first 2 months or so they hung out with us in the living room because we would not let them hang out in his room. Eventually we caved in since he kept asking and said only with his door open can they go inside the room. Yesterday I picked up clothes off of his floor prior to them hanging out in his room. After they left, I picked up some pants that were essentially soaked in semen (not to be graphic). Possibly used to wipe up afterward not sure about the details.

He now wants to hang out again with her on Thursday but not sure how to communicate all this. I have a feeling he may deny it (I love him, but he does have a tendancy to lie like a lot of teenagers) but I have not started the conversation yet. How do I approach this without seeming very confrontational? What steps should I take?

Another thing is he is with his dad on the weekends and his dad likes to give them space and privacy which adds another layer to this.

Thanks!