I am curious as to how he has managed until now. How did his parents get him registered for a school? Has he got any 'official' documents like a driving licence,( you can't get a driving licence without proving your age by showing your birth certificate.) Does he have any other documents like a National Insurance card or a NHS card and number? If he has none of these how is he working , paying tax or making pension / NI contributions? There is a lot more to not having your birth registered, as officially you do not exist!
Was he born abroad as my daughter was? If so you need to contact the High commission or Embassy or the place of birth.