Why did Mr. Lewis stop for Bud?

The answer to this question can be found in chapter 10. Bud has decided that he is going to walk to Grand Rapids, and he's trying really hard to not be noticed. He hides in a bush or off to the side of the road every time a car passes, but he quickly grows tired of this action.

I was getting so tired that I started to forget to duck in the bushes when a car would roar by.

Bud is noticed by a car that stops and reverses back to Bud, and the man in the car gets out and starts whistling loudly and calling out to Bud. Bud is still hiding, but the man knows what he saw. He yells out that being a "brown boy" outside of Owosso is not the place to be.

And I'll tell you, I've seen some things out of place before and a young brown-skinned boy walking along the road just outside of Owosso, Michigan, at two-thirty in the morning is definitely not where he ought to be. In fact, what is definite is that neither one of us should be out here this time of night.

Bud eventually will get into the car with the man, and the man further explains that Owosso is an unsafe place for black people.

Bud-not-Buddy, you don't know how lucky you are I came through here, some of these Owosso folks used to have a sign hanging along here that said, and I'm going to clean up the language for you, it said, "To Our Negro Friends Who Are Passing Through, Kindly Don't Let the Sun Set on Your Rear End in Owosso!"

Mr. Lewis stops for Bud because he is a kind man, and he knows that Bud is young and in a dangerous location. Mr. Lewis wants to help get Bud to safety.