there may be some records of his business with the Duke in the Atholl Papers (see www.manxnotebook.com/history/ap/index.htm for a brief description) - there is a card index of names appearing in these papers in manx Museum (personal researchers only) and the papers are on microfilm - I've not looked at the later papers myself.
There is also I believe a large number of papers relating to IoM affairs in the library at I think Blair Atholl