My phone number being used by another person with imessage

My fiancé uses an Android and recently changed his phone number. We realized he isn’t receiving messages from iPhones unless they turn off iMessage. I have an iPhone and my iMessages are going to some random person who is no help and is getting mad. How is it even possible that his phone number is being used by another person on iMessage??! I have entered his number on apples website to deregister it from iMessage but we don’t get the text to confirm. I called Apple support and they had no ideas. This is a huge Apple mistake/issue but I can’t get any solutions. I have searched the internet but can’t figure anything else to try. We don’t want to change his number and shouldn’t have to. Any ideas??