Meaning: This sentence provides more information about the relationship between the boy and the dog. The boy owns the dog, and they are playing together in the park.
Choice of Words: The addition of his to the sentence clarifies the relationship between the boy and the dog. His is a possessive pronoun that indicates that the dog belongs to the boy.
The boy and the dog are playing in the park
Meaning: This sentence emphasizes that the action of playing is taking place at the present moment. The boy and the dog are currently engaged in play in the park.
Choice of Words: The use of are instead of play indicates that the action is happening right now.