彼がこの間私たちが話題にした少年です。

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:

  • 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.

  • 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”.