[Mushiro]

Since his Zanpakuto is in a constant released state, it's probable that he's said it's name in the past. Otherwise it would still be sealed in a katana form. The reason he doesn't remember it's name or refuses to call out it's name is another story. I beleive Kenpachi will learn to rely more on his Zanpakuto rather than his Reiatsu, it's inevitable unless he wishes to die. We will most likely be seeing him call out his Zanpakuto's name within the next couple of manga chapters so that he can use Shikai abilities. However, I don't beleive we'll be seeing a Bankai any time soon.