Photo credit: ANGUSalive
There are several portraits of Duncan Forbes of Culloden on Art UK.
I have not found any reference to him being a bishop and perhaps there is no resemblance to the other portraits of Duncan Forbes, even considering the age difference. If the title of bishop is traditional, maybe it's worth considering that there has been confusion in the past. The sitter could then be, for example, Bishop Robert Forbes (1708–1775).
Another possibility might be John Forbes of Culloden (1672–1734), MP and elder brother of Duncan Forbes.
The costume appears to be of the 1720's, which would tend to preclude Bishop Robert Forbes (b.1708) on apparent age grounds. Duncan Forbes was not a Bishop but a judge.His biographer described him thus "above six foot high, very streight and genteel in his body, which much enclined to slenderness; his face was smooth and majestick, his forehead large and graceful, his nose high; his eyes were blue and full of sweetness, and tho' very quick, yet rather grave than sparkling; the pupilla was charmingly intermixed with the white; his cheeks and chin were finely proportioned, his hands and arms were every way delightful.". Does the sitter in the portrait have blue eyes ? - hard to tell from the photograph.
I have tracked Forbes family portraits for five decades.
This figure may well represent a person of that clan; the man’s nose and lips are indeed similar to documented portraits from the era. Yet his common wig, drab clothing, and lack of appurtenances signifying wealth, rank, or office do not indicate a man of importance. That he might have been a baron, Bishop, or judge remains unlikely.
Perhaps a through cleaning may reveal more information. Until then,
“Very possibly a member of the Forbes family” is fair.
I would discourage, though, further attempts to make a specific identification. The Forbes clan is enormous; tracking them is made doubly difficult by their constant repetiton of only a very few first names.
Trying to sort the dozens of Johns, Williams, and Jameses is mind boggling and leads to nothing but frustration and futility.
Believe me, I have tried!
John Hazard Forbes / Thomasville, Georgia