Why is Douglass specific about making friends with "little white boys"

Making friends with "little white boys" became Frederick's path to literacy. They helped teach him to read.

The plan which I adopted, and the one by which I was most successful, was that of making friends of all the little white boys whom I met in the street. As many of these as I could, I converted into teachers. With their kindly aid, obtained at different times and in different places, I finally succeeded in learning to read.

How did Douglass learn how to read when running errands?