Hi Jim,
reaching out as noone else has yet with a few questions:
1) Is this the same Archibald Ritchie that emigrated on 9 March 1948 aboard the Queen Mary? If so, have you tried searching the naturalisation papers for Canadian Immigrants? They only go up to (I think) 1955, and you'll have to search manually by month, but it's a start.
2) What are the names of Archie's parents? Do you know if they moved as well? I ask as I can find an Archie, born in Scotland, in Ottawa in 1921, just not sure if it's your relative. If it is, you might need to expand your search.
3) what date is the last known address? Your best bet is to take this date and look at local newspapers or reach out to any local archives for that area. If they lived in a relatively small community there's a good chance that you'll be able to locate something in the archives.
Hope that helps,
Gemma