This miniature portrait is unsigned and the sitter is unknown, although he is obviously an army officer of some nature.
However, a kind visitor to the website has advised that the portrait is by Jean-Baptiste Joseph Le Tellier (1759->1812). He worked in Paris as a painter of miniatures on ivory, and on enamel, in the years before and after 1800, although his enamels appear to be rare. Seven miniatures by him appear in the Louvre catalogue.
The Louvre catalogue gives his birth date as 1759, but includes as fig. 431 a miniature by him, which is signed and dated 1770. Thus there must be some doubt, either about the date of his birth, or about the signature, as in 1770 he would have been aged only 11.
Schidlof comments that Le Tellier "was inconsistent in the quality of his works, but in his good pieces he showed a first class talent. They often have an appearance which reminds one of Dumont. However, it cannot be said that he imitated Dumont, for his much finer and more detailed technique differs entirely." 703