Meaning: This sentence describes the boy's feeling of satisfaction and happiness.
Choice of Words: "The boy" is the subject, "smiled" is the verb describing his facial expression, and "contentedly" is an adverb describing his emotional state.
Alternative Expressions
The young lad beamed with joy.
Meaning: This means that the boy was very happy and excited.
Choice of Words: The young lad is a boy. Beamed means to smile widely and brightly. Joy means a feeling of great happiness.
The boy's face lit up with a big grin.
Meaning: This means that the boy was very happy and smiling broadly.
Choice of Words: The boy's face is the front part of the boy's head. Lit up means to become bright or illuminated. Grin means a wide smile.