This portrait is of Robert Orme (1725? - 1790) who transferred to the Coldstream Guards in 1745, and was promoted to lieutenant in 1751. He was aide-de-camp to General Braddock, and was wounded when Braddock was killed by the French (near Pittsburgh) in 1755. He returned to England and resigned from the army in 1756. He is recorded as a lieutenant on resignation, but may have been a temporary captain under General Braddock, certainly he was referred to as Captain Orme.
The composition is based on a sketch of 1752 from 'Saint Francis embracing a Sick Man' by Jacopo Ligozzi in the Cloister of the Ognissanti, Florence.