who was the Prince Regent's black servant?

In 1787  Sir Joshua Reynolds exhibited a major portrait of the Prince of Wales [later Geo IV]  attended by valet who is shown fastening the prince's belt. 

This was the premier picture at that year's Royal Academy in May.  It is visible behind sitter and artist at the Summer Exhibition opening in May that year. A bit hard to see amidst crowd of persons and pictures [and dogs] but very clearly centre of the display, as befit the royal figure.

As it happened, the campaign to abolish the transatlantic slave trade had just been launched.  Reynolds was a supporter and signatory to the first of many petitions, as the campaigners were not successful until 1807.

So who was the manservant depicted?    in a good number of earlier aristocratic portraits, attendants of African appearance are often included as exotic accessories, some of  which will have been painted from life, others 'copied in' from unrelated studies.  Given the Prince of Wales' prominence at this period, and the nature of London 'society' centred on the Court, it seems unlikely that Reynolds' portrait featured a generic valet.  Surely this young man was one of the Prince's attendants? 

Letters from Reynolds to the Prince of Wales's accountant show the portrait was still for sale a year later.  'The only Picture that he[Reynolds] has finished of His Royal Highness since the account sent in the last year, is a whole length with a black servant, which is still at his house, the price of that is two hundred Guineas',  reads the letter of 22 April 1788 to Mr Robinson. 'There is another whole length of the same price which is not quite finished.'   Perhaps the prince had inquired about purchasing it, though he did not. .

Then on 24 April Reynolds wrote again: 'There is another whole length which is intended for Lord Charlemont which according to Sir Thomas Dundas's opinion ought to be copied from the picture  of H.R.H with the black servant, and to have one sitting to make it an original.'  

Do these references imply that the 'black servant' was an employee in the royal househoold?  

Is it possible to find out who he was?