Arty, you posted whilst I was typing. Your son is 24. He's an adult. He can choose to use whatever surname he wants.In fact, his choice is easy because all he wants to do is use the name which was registered as his surname on his birth certificate. He'll have to get an official copy of his birth certificate and explain to the authorities what has happened. They may not want more (e.g a deed poll) in the circumstances viz the name he wants recorded is his legal name as shown by the birth certificate and, assuming he was a child at the time, his mother had not got your consent nor any court order ,but, if they do, a deed poll is no more than a solemn declaration that henceforth he wishes to be known by the name of [whatever his birth name is]. A solicitor will do one for him to sign and have it witnessed by someone in the solicitor's office.