Question on Resetting an Apple Watch

Hi. My kiddo has an Apple Watch that he reset for his younger sibling. It asks him to enter his Apple ID and Password when he tries to set it up on the other phone. Problem is he cannot remember the email address he used to set up the iCloud account when he got the watch. Is there a way for him to get the email address he used from Apple? Perhaps by contacting them with his new Apple ID?

Thanks in advance for any help.