Hi,
You can definitely rule out the RCAF service number. It was not RCAF policy to grant Canadian service numbers to RAF fellows who came over for training.
It is also a misconception that the RAF service number is mandatory to obtain a service record.
On their site they merely ask you to fill out the forms as completely as possible:
http://veterans-uk.info/service_records/service_records.html(Look for the RAF ones as the link also has army and RN)
There is a £30.00 charge for them to search for the records and they make it explicit that it is non-refundable.
Should you decide to go that route more questions emerge. Are you his next of kin? Or is your grandmother and/or parents still with us?
If he has been deceased 25 years or more, no need for next of kin permission is required.
The only other option I see, and you haven't given anything but years, would be to locate which draft of men headed for Goderich (a dreadful place - weatherwise, in winter) and you could see what is held by the Library and Archives Canada in Ottawa.
It's all complicated to a degree but do not give up hope. It's not impossible.