He asked to be exclusive but still opens the app: what to do

Start by being honest about how often this happens and what it looks like. One time he opened the app while you were sitting next to him is a different situation from him sending messages on it every evening. Look at whether he has stopped doing things that signalled interest in you earlier, like making plans, replying quickly, or bringing you into his week. If his effort is still steady and the app use is occasional, it may be habit. If the effort has thinned out at the same time the app use shows up, the two are connected.

Also notice how he reacts when the app comes up even indirectly. Relaxed people usually say something like "oh I haven't deleted it, I should" and move on. Defensive people change the subject, get annoyed that you looked, or turn it around onto you for checking. Neither reaction proves anything alone, but together they tell you how comfortable he is with the conversation you have not had yet.

Your reply has to make the conversation possible without turning it into a trial. Say what you saw or what you noticed in one or two plain sentences, and say how it sits with you given what he told you. Keep the evidence small. Do not list every time you suspect something, because a long list makes him defend the list instead of answering the question.

Then ask what exclusivity means to him in terms you can both picture, such as whether he has stopped using dating apps, whether he is still talking to other people, and what he wants to do if he is bored or curious. Let him answer fully. Your job in this moment is to hear what he says and decide afterwards whether it fits what you need, not to win the argument while he is still talking.

After you have talked, leave space before you read anything into his next few days. People who have been caught off guard often get quiet or a bit cagey while they think, and that is different from avoiding you. Watch whether he comes back to the topic on his own, whether he is warmer or colder in ordinary ways, and whether his plans with you continue as normal.

If he gives a clear answer, like deleting the app or saying he has been using it out of habit and will stop, see whether his behaviour follows over the next couple of weeks. If he dodges, minimises, or gets angry when you ask again, that is your answer about how he handles this kind of thing. You are allowed to decide the relationship is not working on that basis, even without proof of anything else.