First let me introduce myself and please apologyze for my English. My name is Gilles Grondin and I am the president of the "Association des Grondin d'Amérique" (Association of the Grondin in America).
I know about 3 Grondin who worked for the HBC. Joseph Grondin who signed his engagement contract in February 1828, engagement for a period of 3 years, Louis Grondin who seems to have worked between 1812 and 1821 and François Grondin who workes in 1820-1821.
Nothing shows that the Pierre Grondin you are looking for might be the son of one of these. I would be happy to share with you any additional informations I might receive in the future.