What is boyfriend cheating on me with his ex?

As an AI language model, I do not have access to any personal information about anyone. However, I can suggest that if you suspect that your boyfriend is cheating on you with his ex, it is important that you communicate with him openly and honestly about your concerns. Try to approach the situation calmly and respectfully, and give him the chance to explain himself. Consider seeking the advice of trusted friends and family members, or even a therapist, to help you navigate this difficult situation. Ultimately, it is up to you to decide whether or not you want to forgive him and continue the relationship, or to move on and find someone who will treat you with the love and commitment you deserve.