"Why is this boy crying?"

Translation: Dlaczego ten chłopiec płacze?

[Annaphoric]

Can it also be "Dlaczego płacze ten chłopiec?"

[Jellei]

Maybe in a song, or poetry. It doesn't seem like a natural word order. You normally ask about the 'crying' part, and your question emphasises 'this boy'.

[Xamik]

Add "czemu". Only "dlaczego" is accepted.

[Jellei]

That's surprising. Added now, of course.