Whoever he is, he’s back. Exactly a hundred years ago, The Blue Boy, Thomas Gainsborough’s full-length portrait of about 1770, was honoured with a three-week valedictory display at the National Gallery, after the Duke of Westminster sold it to the American tycoon Henry E. Huntington for a record price. Ninety thousand people turned up to bid this much-loved masterpiece of British art farewell, touched, perhaps, by its vision of innocent youth when the losses of the First World War were still raw. When the painting was finally packaged up, the gallery’s director wrote “au revoir” in pencil on its stretcher, before it departed on an ocean liner for California, where it has hung ever since.

Now, at last, that wish-like inscription, which remains legible today, has come true. Towering over a darkling landscape, and still resplendent in a shimmering blue-satin costume evoking glamorous portraits by the 17th-century Flemish painter and court painter to Charles I, Anthony van Dyck, Gainsborough’s Blue Boy returns for 16 weeks to Trafalgar Square.

Like a cordoned-off VIP, he gets his own private room near the grand entrance, where, staring at us self-confidently, one hand upon a hip, he occupies an entire wall by himself, accompanied by an entourage of four comparable works by Gainsborough and Van Dyck. For all the time that has elapsed since his last appearance on this stage, we still don’t know his identity. When, in 1770, the picture was exhibited at the Royal Academy, it was described only as “Portrait of a young gentleman”.

A few minutes, though, in this anonymous model’s company reveals details that, having never visited The Huntington in San Marino (which, until now, hasn’t lent the work), I’ll confess had passed me by. The thickness of the white paint delineating the tasselled edge of his tunic. The deliberately casual treatment of the reflections upon the satin: ghostly lightning bolts zigzagging through the blue. The brightening sky on either side of his torso, giving the impression that he’s sprouting angel’s wings. The glossy intensity of those red lips, echoed only in a half-glimpsed sunset, and the smudgy rouge of his cheeks. He’s certainly pretty, this long-lashed little chap, but not effete. Beneath his ears, brown curls frolic, evoking the puckish energy of boyhood.