Transparency between us has always been a written part of our rules. He knows he is supposed to tell me when he is talking to someone new, or reconnecting with someone significant from his past. And yet he has been keeping this a secret for weeks. This upsets me a lot because we are just getting back on track in our relationship after he broke a promise he made this summer (with a different partner), and compromised my trust in him.
You could choose not to "confront" him about it, but choose to "call him into account" instead. You have a written agreement he is not meeting. HOW you call him into account doesn't have to be confrontational in tone, but calling into account is calling into account. You want to know and clarify something. You are giving him opportunity to explain.
He cannot be a mind reader. You could let him know your feelings are upset, your trust in him is shaken up, and you worry about follow through on his promises. Could he please explain himself?
And see what happens.
You own your baggage, he owns his baggage, and you deal with what is in front of you in as calm a way possible because the relationship needs tending and care.