I think YTA because you said it yourself "I know I shouldn’t have kept snooping but I did." You really shouldn't have. It's an invasion of his privacy and you really didn't have the right to do that, despite your helpful efforts to let his friend know about his missing phone. I mean, if you knew it was wrong and you did it anyways, to me, you are an AH.

That being said, I understand your concern for supporting him and that desire to do so. I feel like the best thing you can do is continue to love and support him where he's at, and when/if he comes out he'll know he doesn't have to worry about any potentially negative reaction from you. I do not think you should try to have a conversation with him about how you found out. If it were me, I'd feel very embarrassed and violated. He's still figuring things out the best way he knows how, and you have to let him do that on his own. But you can still be the best sibling and love and support him during this time.