Ex partner has got control of my Apple ID

My son has just had a messy split from his partner! The partner has an old iPhone that he had ! She has gained access to his Apple ID and changed contact phone number to a mobile she has! He wants to get control back of his account and shut the iPhone down that his ex partner has now got! He is still paying for the account via his bank card! He doesn’t want to loose his music and other stuff stored on iCloud ! He cannot get into account as reset details will be sent to her mobile! Can anyone suggest a suitable approach ?