I think my underage son might be having sex with an adult. Help?

Throwaway for obvious reasonsBackground: Me and husband live in Arizona we have a daughter and a son. Our son is 15 years old and came out to everyone a several weeks ago. We support our son and talk to him frequently. When he came out he said he had “been with girls” but had not had sex but he said that he would try to now with boys. We guessed this and told him to be safe which he agreed to do. A few weeks ago our neighbor hosted a BBQ where my family was invited. The whole neighborhood was invited including Robert(Not is real name) a college freshman who lives one house over from us. My husband and I noticed Robert talked to our son the whole time which at first was fine, I was glad our son was making friends. However over the past few weeks I’v noticed that my son has been constantly texting Robert all the time, even late into the night. My son also started to spend a lot of time after school at Robert’s house. All of these things seemed a little weird but totally fine. A few days ago our son was sitting in the living room with me and my husband texting on his phone. I asked who he was texting and he said Robert. Abruptly he got up and went to his room without saying a word. I then got up to see if something was wrong but when I came into my sons room I found him taking a picture of penis. He was freaked out and said get out so I did. When he came out of his room me and my husband talked to him about how he shouldn’t send naked pictures, we grounded him and took his phone. I don’t know his password so I couldn’t look but I kept his phone in my pocket so I could see and new incoming texts. Later that night Robert texting saying “Just come over” at 1am in the morning. Two days when I was doing my sons laundry I found a used condom wrapper. I think the evidence is damning that Robert who is 18 is having sex with my son who is 15. From what I understand its legal since Robert and my son are only three years apart but I still think its wrong. Robert is an adult in college our son is a sophomore in high school. I'm fine with my son experimenting with boys his age its just I find what Robert is doing is wrong. Me and my husband don’t know what to do. My husband thinks we are overstepping but I think whats happening is wrong. I feel like a fish out of water so I came here for help. Thoughts? What should I do? via /r/Parenting http://ift.tt/2pfvZgM