George's father was Abraham Dupuis an American merchant of Gracechurch St and Fulham who died in 1777. Abraham's brother was Anthony Wagner of Pall Mall, hatter to King George II. Their father was Abraham Dupuis also of Pall Mall, Westminster, Court Milliner to the Dowager Princess of Wales and he died in 1737.
It is thought that in this case 'American merchant' means trading with America.