I understand. You might want to apply for a certificate confirming right of abode.
Please see the following page:
You will find the following text:
If you were a citizen of the United Kingdom and Colonies and were ordinarily resident in the UK for a continuous period of 5 years before 1 January 1983, and were settled in the UK at the end of that period
You should send evidence of:
your citizenship of the United Kingdom and Colonies, such as a passport, or certificate of naturalisation or registration; and
your settlement and 5 years of ordinary residence in the UK before 1983, such as your passport, P60 tax forms, details of National Insurance contributions, social security claims or employers' letters.
This would seem to suggest that if right of abode is not indicated in the passport then a certificate confirming this can be obtained if he was a CUKC (he was) and he had lived in the UK for at least 5 years prior to January 1983 (he had). It might be worth applying for right of abode confirmation before applying for the passport.
Sorry to use rather vague terminology but these situations are never as clear as we would like them to be.