Okay so everyone knows Kenpachi does not know the name of his zanpakuto so here is my theory.
When he first manifested his zanpakuto he met someone(who would obviously be his zanpakuto spirit) and they told him their name and he came to admire them. When he became too strong he couldn't find that person anymore and became the homicidal S.O.B we all love. He eventually came across young Yachiru who he name after the person he admired.
I can support this in two ways:

1) His zanpakuto may be refusing to tell him it's name again and not not being able to tell him.
2) most shinigami met/learn the name of their zanpakuto before manifesting it. It's possible Kenpachi did the same and never realized it.

So lets her you're opinions.