When a guy says "I'm his," it typically means that he considers you to be in a committed and exclusive romantic relationship with him. The phrase suggests that you belong to him in an emotional and romantic sense, and he sees you as his partner or significant other. It's a declaration of possession and commitment.
It's important to note that communication in relationships is essential, and people may use different language to express their feelings and intentions. If you're uncertain about the meaning behind his words or if you have any questions about the nature of your relationship, it's best to have an open and honest conversation with him to clarify your status and ensure that you both are on the same page.