This Handsome Young Man . . .

 . . . is Giuliano de Medici, brother to Lorenzo.  This portrait, by Botticelli, hangs in the National Gallery of Art in Washington, D.C., and was most likely done posthumously, as Giuliano was murdered in the Duomo in Florence, on Sunday, April 26, 1478 in the Pazzi Conspiracy (more later on that).

     At the time of his death, Giuliano was no longer enamored of Simonetta Vespucci, as she herself had died exactly two years earlier, on April 26, 1476.  Giuliano had taken as his lover Fioretta Gorini, the daughter of a tradesman.  Fortunately for the Medici family, Giuliano had taken some of his friends into his confidence, and after his death, it was revealed to Lorenzo that Giuliano had had a love affair which was to result in the birth of a boy on May 26, 1478.   Lorenzo reportedly sought out Fioretta, and "without much difficulty, persuaded her to let him have the rearing of the child."  (per David Loth, in Lorenzo the Magnificent)

     The child was named Giulio, and was raised along side the other children of Lorenzo and Clarice.  Giulio was later to become Pope Clement VII.