Isaac Columbus and Angelique Dupont were originally from Quebec (married 1796), but emigrated to Toronto prior to 1804. They had 13 children, including Isaac, Louis, Harriet, Angelique, John (Jean) and others. I am descended from one of these children, born circa 1817 (from census records). Other children were born earlier, including one known to have been born in 1811.
The family name was also spelled Coulombe/Colombe. The Tanguay pages on the Web (try to the Quebec GenWeb site) trace the family line back a considerable period. Isaac (who married Angelique) is descended from a Louis who emigrated from France to New France in the 1660s. Louis was from the Normandy area of France, where the story was well-known that the family was believed to have descended from Christopher Columbus.
Isaac, Angelique and family were some of the original settlers of Toronto. Isaac's profession was gunsmith/armourer. Certain of Isaac's children subsequently emigrated to the Penetang area of Ontario--Drummond Island, for example, while others emigrated to southern Ontario.
If your Alexander was born 1789, he is not likely to be one of Isaac/Angelique's children, but there is the possibility of another family relationship (brother to Isaac? first cousin?). I would check the Tanguay pages--it will take some time to familiarize yourself with the format, but there is extensive "Help" available on-line to help you decipher the information.
Was a death certificate available for your Alexander? (Just wondering, as it might indicate parent(s).)
Hope this helps.