RCAF would be the Air Force.
Are you sure it is RCAS? If you are able, scan the details of the certificate and post it here. Some script can be hard to decipher and someone else may see what it is.
Looking at an index of Canadian Regiments etc., it is just possible that he was in the RCASC. There again, it could be that he
was in the Armoured Corps and the last letter was missed or has faded or been obliterated. But this is only my thoughts.
As for regimental numbers, I am sure there was a thread somewhere on here that gave some info on that; I believe that the initial letter stood for the area where enlistment took place, but I could be wrong.
Paul.