Right of Abode means what it says- your husband has the right to live, work, study etc. in the United Kingdom with all the rights of a British citizen except a British passport. Also the right to vote. He has this as a Commonwealth country (Canadian) citizen adopted by British born parents before 1983. It cannot be lost as long as he remains a Canadian citizen, although in recent years the Government has tightened up on the evidence requirements to prove Right of Abode. He will need a new stamp in order to use his status.
IF he was added to his mother's British passport, it should mean he was registered as British (only British nationals should be included on British passports)- although not necessarily. The procedures for adding children to British passports in the past were not as strict as they are today.
If he was registered as a Citizen of the U.K. and Colonies as a child, this could have been done at either the British High Commission in Ottawa (or Canberra) or at the Home Office, if it was done after returning to the U.K. Pre-1986 Home Office records may be available from the National Archives.
https://www.gov.uk/get-replacement-citi ... ertificate
If he was registered at one of the High Commissions- there is no information available as to what happened to the pre-1983 citizenship records. You may be able to gain some further details by writing to the Foreign Office in London.
And if there is no record of him being granted British citizenship- then he may apply for naturalisation as a British citizen, if he meets the normal residence requirements.
This is not intended to be legal or professional advice in any jurisdiction.