. . . . . . . . young boy was roaming here and there.

Indefinite articles 'a' or 'an' are used with nouns which are indefinite or general. They are used in a vague sense of a certain like to state a certain member of a group or class. Here in the given question, 'boy' denotes a group or class of people, therefore use of the indefinite article is appropriate. The choice between 'an' and 'a' is determined by the sound of the noun. Before a noun beginning with a vowel sound 'an' is used.