In the book Tuck Everlasting why does the man in the yellow suit go to the Fosters' house?

The Man in the Yellow Suit goes to the Fosters’ house to find out about the source of the spring.

The Man in the Yellow Suit is chasing a lead.  He believes that the spring originated somewhere in the area.  He wants to find the spring to exploit it for profit.  He is basically looking for any information from the locals that might prove that the story he has heard is true.

Winnie and her grandmother are not being very receptive to his questions, but then they hear the music from the woods.  Winnie’s grandmother forgets to be cautious and tells the man that the music is from elves.  He gets excited when he realizes that she has heard the music before.

Then the moon rose. The man came to himself and sighed. His expression was one of intense satisfaction. … Then he turned and disappeared down the shadowy road, and as he went he whistled, very softly, the tinkling little melody from the wood. (Ch. 4)

The man believes that he has the source of the spring.  The music clearly means something to him, and so does the fact that Winnie’s grandmother has heard it before.  It is a mysterious sound, and to him it means he is on the right track.

That music comes from Mae Tuck’s music box.  It is one of her most prized possessions. It is clear that Mae has played it near the spring before.  It comforts Mae, and she plays the music absentmindedly when she is distressed.  She tells Winnie that she didn’t realize that others could hear.

When he does find the Tucks, the Man in the Yellow Suit explains why he was fascinated with them based on stories his grandmother told him.

I was fascinated by my grandmother's stories. People who never grew older!  It was fantastic. It took possession of me. I decided to devote my life to finding out if it could be true, and if so, how and why. (Ch. 19)

Of course, the Man in the Yellow Suit is a threat to the Tuck family.  They do not want anyone to know that they live forever.  They also do not want anyone to know about the spring, and the man wants to sell the knowledge.  This turns into disaster when Mae accidentally kills him.  The constable says that she will go to the gallows if the man dies.  This is foreshadowing because she is arrested and should be hanged, but since she would not die, the secret would get out if they tried.  This is why Winnie has to make the choice to break her out.