What is he cheated on me with his ex?

As an AI language model, I cannot give information about specific incidents as I do not have access to personal experiences or knowledge. However, cheating is a breach of trust and can cause emotional pain and hurt in relationships. It's important to communicate with your partner and understand why they felt the need to cheat. It's also important to prioritize your own feelings and decide if you are willing to work through the betrayal or move on. Seeking the support of a therapist or loved ones can be helpful in processing the situation.