English Translation
He is the boy of whom we spoke the other day.
Unnatural word-for-word translation…
この前、私たちが話題にしていたのはあの少年 (or この少年) です。
is much better for the following reasons:
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この間(あいだ) sounds unnatural. Replace it with この前(まえ). この間 and この前 are synonyms, But it’s just a matter of collocation.
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話題にした suggests that the boy is no longer subject to our attention. It’s done. That’s the connotation of した tense. 話題にしていた, on the other hand, is not a past progressive form. But it means that “we brought up the topic about the boy in the past, and we are STILL interested in discussing about him”.