He said yes to exclusive, then went back to his ex. What now?

If he went quiet after you raised it, or answered with a shrug, pay attention to that before you read any meaning into the words he used. A one line reply like 'it's nothing' is a non answer, and a non answer is information. You are not looking for him to say the perfect thing. You are checking whether he will engage with a clear question at all.

Also look at what has changed in his behaviour. Has he gone quieter with you while being active with her? Are the conversations with her late, warm, or about the two of you? You do not need to prove anything with these details. They just help you decide how much weight to give his explanation.

The temptation is to ask follow up questions about her, about why they talked, about what they said. Those questions feel like information gathering, but they turn the conversation into a debate about her and make you look like you are auditing him. Start with what you need to know from him: whether he sees this as a problem with the agreement and what he wants from here.

If he does not see it as a problem, say so plainly and say what the agreement meant to you. Then let him answer. You will learn more from how he responds to a direct statement than from any amount of questioning about the ex.

Send the message at a time when you are calm and he is likely to be awake and free, not in the middle of the night after you have been stewing on it. A late message written in a rush tends to carry more heat than you meant, and it gives him an easy reason to dismiss it as you being dramatic.

If you do not hear back within a day, send one short follow up that refers to the first message and asks if he has had a chance to think about it. If another day passes with nothing, or he responds with a joke that avoids the point, that is your answer about how seriously he is taking the agreement. Stop chasing at that stage and decide what you want to do next.