I would suggest you to hold this for couple of days and let him settle down after that you can ask him about it. However it's all in the past. I understand it's surprising for you to gulp it but we are in a generation where people have couple of relationships before they eventually find the right one. Don't bother about it too much. But just make sure that if it's a past it should be a past in future too. Shall not come and jeopardize your present and future.