One identifying feature of Malcolm Cameron's land petition was the mention of his brother-in-law, Henry Glass. According to this source:
"Henry Glass (fl. 1820-1828) was a general merchant and magistrate in Lanark County, Upper Canada during the early 19th century. Henry Glass was a merchant in the villages of Lanark and Perth in Lanark County, Ontario. He moved to the village of Lanark around 1820, and by the end of that year he had opened a store in the village. In 1821 he married Christena Cameron. By 1823 he had moved to Perth. In that year Perth became a judicial seat and Glass was appointed a magistrate. In 1828 he formed a partnership with his brother-in-law Malcolm Cameron, which lasted only a year. He continued as a general merchant in Perth, however."
This is the biography of the Malcolm Cameron whose brother-in-law was Henry Glass. He's not our Malcolm Cameron, but in researching the Cameron clan, I'm hesitant to rule out a connection, especially since Ewen Cameron's parents have yet to be determined.
Thanks Michael for bringing that land petition to my attention!