How did the man in the yellow suit knows about the spring water and about living forever?

The man in the yellow suit learned about the spring water and its life-extending properties from the Tuck family, who had discovered the fountain of youth in the woods. He had been searching for the Tucks, motivated by a desire to exploit the water for profit and power. His knowledge of the spring's effects came from their stories and his own interest in immortality, which he sought to control for his own gain.