Most Helpful Guy
I believe in second chances. You should not worry and just get back together.
Most Helpful Girl
Honestly, the fact that he told you about the app at all earned trust in my book. You are broken up, he didn't have to tell you anything at all. I think what he meant is that he gets on and looks and perhaps chats, but had no intentions of meeting up to this point? That's my guess.
Anyway, the real question should be, have those issues that caused the break up in the first place, resolved? If not, then you shouldn't get back together. If so, I don't see his dating app as a reason to stop you. Unless however, he keeps getting on it once you're back together.