No tengo ni idea de lo que estará pensando ese chico.

why the future “estará” here instead of “esté”?

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In short, because you’re describing something real (i.e. not knowing something), not an opinion, or an emotion or a hypothetical.

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makes sense, but why the future tense instead of present?

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That’s a way of expressing conjecture or supposition.

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By the way, this is an indirect question; compare with “¿Qué estará pensando?”. Using the subjunctive would just not make sense.

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