He could be a Strathspey Grant. Those Grants in Glenmoriston, Duldreggan, Craskie, Corrimony, Sheuglie and Urquhart are somerhow related to ones in Straspey.
Not all Grants in Charlottenburg were strict Jacobites. Yes they may have leaned that way but did not take part in the 1745 battles as some Grants from Glenmoriston and Urquhart were Presbyterian not Roman Catholic as the Prince was. They would have liked the Prince to regain the throne but they did not want to return to Roman Catholic faith.
I believe that son John Grant is listed just before the father, Peter Grant in the KKR NY book that I have.