He was a professional model. Fusco is a fairly uncommon name, but there was a family of that name based in Bradford, and later Edinburgh. For instance, in the 1881 census two Italian-born British subjects, Benedetto Fusco, aged 30, and Michelangelo Fusco, aged 23, travelling musicians, were recorded in Bradford, Yorkshire.
He posed for a drypoint, Fusco , in 1872. He may have been a model at life-classes organised by Victor Barthe (b. ca 1839), which Whistler attended in the 1870s.
According to Walter Greaves, Whistler made nude studies when working on the portrait Arrangement in Black: Portrait of F. R. Leyland y097, in the early 1870s. Greaves remembered that Whistler kept rubbing the painting down and eventually got 'a well-known Italian model named Fosco to pose nude for the figure.' It is probable that the name was Fusco rather than Fosco.
UK census 1881. Walter and H. Greaves (Pupils of Whistler), Goupil Gallery, London, 1922, pp. 19-20.
Margaret F. MacDonald, Grischka Petri, Meg Hausberg, and Joanna Meacock, James McNeill Whistler: The Etchings, a catalogue raisonné, University of Glasgow, 2012, online website (cat. no. 106).