Seems a bit pointless lying about a one year difference, just got to hope he doesn't take it too badly
I understand it's a small difference but I think it's the legal factors that make it important, is it not? Although, I did the same thing when I was 17. It's not much of a confession lol.
Message him and tell him that you need to tell him something; then tell him that you’re actually 17 and you’re sorry that you lied, explain why you lied (idk you were too young to use your dating app). There aren’t any legal issues and I doubt he’ll mind but make sure you tell him now because it’ll be a lot worse if you tell him 6 months in.