I would recommend just telling him that you've realised you don't want to date him again and have decided to move on. And if he asks why, just say that you have moved on.

You don't have to say what you found, you don't have to say anything, he doesn't need a specific reason, because deciding to move on is a reason.

It didn't work, you don't want to try again. Done.

But if you don't tell him something, he's going to keep trying. I think it's kinder to both of you, to say something. Over message is fine, you don't need to call or anything, and you don't need to block or be unkind or whatever. You've decided to move on.