That's a different meaning for the same kanji.
When used to address someone (saying "you") 君 is read as "kimi" and is almost always used to address girls.
If you put it behind someone's name, 君 is read as "kun", and used like written in the "Referring to others by name" section of this page.
太郎君 is always read as Tarou-kun, and has nothing to do with the "kimi" version of the same kanji.
That's a different meaning
That's a different meaning for the same kanji.
When used to address someone (saying "you") 君 is read as "kimi" and is almost always used to address girls.
If you put it behind someone's name, 君 is read as "kun", and used like written in the "Referring to others by name" section of this page.
太郎君 is always read as Tarou-kun, and has nothing to do with the "kimi" version of the same kanji.