Maya: Creating a mouth blendshape from a Jaw Bone



I have a rigged character of a cartoon man. To open and close the characters eyes blendshapes have been used. Easy peasy.

However, for the mouth, the only way to open/close is by clicking and rotating the Jaw Bone.

What would be the simplest way to creating a new blendshape of the mouth opening/closing? - preview of character here.

Thanks in advance,