Simple. His model is terrible.

As you can see with the attached image. The polygons are terrible and cause issues such as that when attempting to pose his neck.

His arms are poorly made.

This is his default pose. A simple A pose. Even then his arms clip into his body. This can even be seen in Slendytubbies 3. It's such a major clipping issue and looks absolutely terrible.

I might still see about adding him into Night 6, depends if I can get his model into the proper poses without him looking too bad.