What should I do about my son being sexually active with an older guy?

I am using a throwaway here because I am not comfortable sharing this. I have an 18 year old son. He's been away often, and he refuses to tell me why. Like 3x a week he would leave the house and wouldn't tell me why. I got suspicious, so went to his room when he was out, and checked his laptop, got access to his social media and found messages between him and some older man.

It turns out, it's his friends parents mate and they been seeing each other for nearly a month and his first message to my son is just 3 says after he turned 18!

The worst thing is he's 56! I can't believe my son would be into someone so old. He invited him to his house and seduced my son from what I can tell from the messages.

I spoke to my son about it, and he had a go at me for snooping, but I told him the man is using him, and it's making me sick. He is unhappy about it but he didn't argue back so I guess he knows it's wrong?

What can I do now?