Deep breath. I answer as a gay man who knew he was gay at an early age.
Your son is probably questioning everything that is currently going on in his life and that is fine. He also needs to know that he is loved and accepted for who he is, whoever that may be. So could you start there, with some reassurance; that I would think he may need.
The huge change in technology makes it very easy for people to be in touch with each other, but I think you suspect the danger here is not about his first relationship but rather about him being taken advantage of due to his age. I fully appreciate that fear. I think you need to share with him after your reassurance, your fear. And explain that this fear comes from love.
I don't know where you live, but when I was much younger I went to a gay youth group that had an upper age range of 25. It was through this group of people that I went to my first bar etc, and it was a supportive group and some of those people I am still in contact with some (cough) 30 years later.
He may have left the phone unlocked accidently on purpose and personally I wouldn't own up to seeing the messages. If he asks you out right tell him, but his privacy will be very important to him now.
He is at the start of his adult life and this will be a huge journey for him and you. Take him to his favourite place and just chat. Don't think you will get out everything you need to say in one go. Be the calm to his internal storm. I wish you and he all the very best.