You would not want to share that you had invaded his privacy as that will cause mistrust on his part.
He may not feel ready to tell you because he may have not had any sexual experience yet. Having feelings or curiosity is still a new territory to him. Perhaps he is trying to confirm that these are valid feelings within himself and that is the lifestyle he would like to live. At 17, his ego is still developing and at this stage the person is quite ego centric. He may feel that his privacy is something that he has control over and hence, had not yet disclosed this fact to you.
You may want to casually inquire if he is dating anyone or interested in anyone. Then perhaps you'd like to mention that safe sex is important in any relationship (heterosexual, gay, one night stands, etc) Let him know that you're there for him and leave it to him to decide when and how to "come out."
Absolutely address the safety issue. Let him know that you have found these dating sites on your computer and that it is not safe to seek relationship with older individuals. You never know what kind of individuals will show up (his judgment in that respect and misrepresenting his age is poor) Have him explain the fact that the websites were visited and then the main focus would be the safety issue versus his sexual preference.
It does not have to be the case where he is attracted to your significant other. If you or someone else had been having access to the laptop, then it would be easier to point out what you have found out. It is also nothing that would have manifested as a result of the move or decreased interaction.
You can just tell him that it had come to your attention that such and such sites were visited (especially when you said one of the sites requires a person to be 21+ yol)
He may get mad or deny it but safety always comes first.
If you believe that your significant other would back you up, first confront your son privately and if he does not accept responsibility, the next time your partner is there, have them also back you up. Focus on the safety issue (because your son may feel embarrassed at first)