Learn Spanish (Latin America): El niño es de México.

Because el without an accent mark means "the", which is used here to say "the boy" (el niño). If it had an accent mark (él), it would mean the subject pronoun "he".

Why do we use es instead of eres or soy?

Es is the form of the verb ser (to be) that is used for "he", "she", or "it". Because the subject here is el niño (the boy), which matches "he", we use es. You would use soy for "I" and eres for "you".

How does Spanish express where someone is from?

Spanish uses the verb ser (to be) followed by the word de (of/from). To say "is from Mexico", you simply say es de México.

Could we drop the subject in this sentence like we do with yo and tú?

Yes! If the person you are talking to already knows you are discussing the boy, you can drop el niño completely and just say Es de México. The verb es already tells the listener you are talking about a "he", "she", or "it".