Boswell met a Colonel Scott of the Guards, natural son to the late Duke of Buccleuch in The Hague, together with William Nairne and Andrew Stuart on June 1, 1764. Presumably, the colonel was a son of the 2nd Duke of Buccleuch, Francis Scott (1695-1751). I have hitherto been unable to identify him any further.
Life with Boswell:
Boswell spent most of June 1, 1764, in the company of Scott, Andrew Stuart and William Nairne, going with them (in a coach) from The Hague to Delft and Rotterdam. They spent the night at the Maréchal de Turenne, with Boswell and Scott sharing a room. Boswell described Scott as "a fine, gay, hearty fellow, quite English and happy".