When the children in the swamp clearing first told Geralt about Johnny, the witcher had every reason to suppose no such person actually existed. It was difficult to imagine anyone living in such inhospitable surroundings without quickly becoming food for drowners or water hags.
Geralt thus suspected Johnny was the figment of childish imaginations, an imaginary friend for lonely orphans. Long years of experience, however, told him not to ignore any possible lead, so he decided to search the nearby swampland for any additional signs of Johnny.
It turned out the orphans were not lying-Johnny really did exist. He was not a human child but a godling, one of a rare breed of creatures that can be found scattered about the Continents wildernesses.
Unfortunately Johnny had lost his voice in circumstances he obviously could not explain, and so neither could he provide much in the way of answers to Geralt's questions.
Once Geralt had helped him regain his voice, however, he talked up a veritable storm, giving the lie to all the tales of these legendary beings' supposed shyness. Johnny also agreed to convince Gran to put Geralt in touch with the Ladies of the Wood.