-kun is used as an honorific suffix usually for boys but can be used for other males with lower or equal status.
I think he's called -kun because his father was also a detective (who died on duty wewt) and he solves his cases with a smile, friendship and effort (being um traditional aspects of a boy?). I dunno, it's a 70s show.
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If you're thinking 2 words btw, you might be mistaking ??? as another, but it also says 'keiji'. It's just furigana to help people like me read the kanji.
Ah, I see. Thank you both very much. And yea, it's a 70's cop show. The intro is pretty damn cool, never seen the full show though. It was on a Sentai TV show trailer compilation DVD.