Summers on the Côte d’Azur are hot and sticky, and the summer of 1952 was no different. And nobody, perhaps, was hotter or stickier than a young man who lay encased in a plaster cast from head to foot. Aged 21, André Villers was suffering from brittle-bone disease and lay in an airless hospital ward near Vallauris, a hillside village above Cannes. He had become so weak that he could not walk.

Confined to bed, staring at the ceiling for countless hours, he knew that life was going on all around him. The wartime terror was over, the Germans had long been defeated, and the countryside was getting back to normal. Even the tourists were returning to the fashionable nearby resorts of Nice and Antibes. How he longed to be normal, to walk the streets once more, to drink a pastis in the cafe and to chat to girls. But for now he had to live inside his head.

He could not know that within a few months he would meet an older man who would change his life and set him on the path to fame. He was Pablo Picasso, at 72 then the world’s most celebrated living artist. Matisse and Chagall, also renowned, were living and painting nearby, but Picasso, who lived in Mougins, was the acknowledged superstar. “It was a unique meeting,” he says. “I loved everything about him.”

Though more than 50 years separated the pair, it was the start of an extraordinary friendship, which would result in a frenzy of artistic collaboration. It began with some iconic photographs of the painter at work – now in museums all over the world – and ended with Picasso posing for him in his underpants. Much of their time together was horseplay, Picasso clowning for the camera, wearing silly hats and fake moustaches. It was a time of Two Go Mad in Mougins.

The results are being shown in Britain for the first time at a new exhibition in London, Bonjour Picasso. Though the odd portrait has appeared in books, Villers’s work has never been seen in its entirety. Though famous in France, Villers, now 75, has been overlooked here. But his support, encouragement and unfailing devotion to the great man helped him at a critical time in his life. France has recently recognised his importance by making him a Chevalier de l’Ordre des Arts et Lettres, while the museum in Mougins is to take his name.

When Villers entered his life, Picasso was between lovers; in the middle of splitting up from Françoise, the mother of his children Claude and Paloma, and taking up with his new young amour, Jacqueline. Picasso’s biographer John Richardson acknowledges Villers’s role: “Picasso was someone who needed to feel loved and supported. And he valued his sensitivity and the fact that he understood his work.”

The two could not have been more opposite. Though in his seventies, Picasso was extrovert, strong and had yet to father his last child. He was rich, his paintings hanging everywhere from the Metropolitan Museum of Art and the United Nations in New York to his home town of Malaga, Spain. Meanwhile, Villers, shy and a recovering invalid, was penniless.

He had been sent to the south for his health as a teenager, and had spent five long years in a sanatorium. Doctors had told his family, living near the Swiss border, that the warm climate and sea breezes would do him good. It was in the hospital, as he made a gradual recovery, regaining the use of his legs, that he took up photography. “Until then I just read all the time. There was only literature when I was bedridden.” The camera opened his eyes and, as he grew stronger, he began taking pictures of children in the village.

It was one spring day in 1953 that he met Picasso, who was living nearby. Though he was world-renowned as an artist of genius, locals referred him to as “le fou”, the madman. “I didn’t know much about him then,” he recalls. “In those days you were considered old when you were 40. So a man of 72 seemed ancient to me.” Picasso asked him about the camera he was carrying, and with uncharacteristic modesty added: “I take pictures too.” He later invited him to his studio.

The young man began by photographing a faun in the studio, a central image in the artist’s work, that Picasso had fashioned out of paper as a découpage, or cutout. When Picasso saw the results, he told him: “I see you have understood me.” After that, he enjoyed carte blanche to do what he wanted. “We didn’t speak much. He would work and I would get out my camera.” Villers was the closest he got to an official photographer.

Theirs was an affectionate relationship. In a play on the French for picture, tableau, Villers called him “Tableau Picasso”. His father had died when he was 10 and Picasso took his place as the years passed. “You are my son,” the painter once told him, “perhaps even more than my own.”

There was a problem, though, with their meeting. Villers was still a patient at the sanatorium, so to visit the studio he had to sneak out. As he acknowledges today, running away from the hospital and trudging up and down the hills of Provence aided his recovery.

Picasso also helped him financially. His camera was so old, the artist said it looked like a “sewing machine”. When it broke, he bought him a new one, and Jacqueline often gave him film.