Learn Spanish (Spain): ¿Quién llama al chico?

This is the personal "a". In Spanish, when the direct object of a verb is a specific person (like el chico), you must put an a in front of it. English doesn't have an equivalent for this, but it acts as a marker to tell us that the boy is the one receiving the action (being called).

Why does the sentence say "al chico" instead of "a el chico"?

In Spanish, whenever the personal a is followed directly by the masculine word for "the" (el), they must combine into a single word: al. You will never see a el written as two separate words.

Could this sentence mean "Who is the boy calling?"

No. Because al chico has the personal a attached to it, we know for sure that the boy is the one being called (the object). If the boy were doing the calling, the personal a would go in front of "who" instead, making the sentence ¿A quién llama el chico? (Who is the boy calling?).

The English translation uses "is calling", but I only see "llama". Shouldn't we use a form of "estar" here?

You don't have to! The simple present tense in Spanish (llama) is very flexible. It can mean both "calls" (a routine) and "is calling" (happening right now). Therefore, ¿Quién llama al chico? perfectly translates to "Who is calling the boy?" without needing any extra verbs.