One update here. After I posted that, I came across the following petition on the UEL listings at Ancestry.com:
John MacDougal , Son in Law to Alex. Grant , deed., appears, saith: The late Alex. Grant was in Sr. John 's Regiment, died in the year '77 , left a Widow & 2 Infant Daughters. The Widow died in '80 . Claimt. married
Catharine , the Eldest Daughter, in '84 , she was under age in '83 .
United Empire Loyalists, Part II Evidence.

So unless he also had a son who had already reached his majority, he's not the father of "my" Alexander.