And this is what I've ended up with. Next I'm going to extend the next down from the head some, but that will probably be the last major modeling change I make to the head.
At this point you should have everything connected so there is just a single hold at the base of the head. If you need to bring part of the back of the head down to connect with the jaw, do that first by selecting the edges at the base of the back, holding down shift, and pulling them down to line up with the rest of the face. Weld any verticies that need to be welded and once you're ready select the edges surrounding the entire opening at the base of the head.
Go into the Left viewport and while holding down Shift, drag down to create new polygons. Do this several times so that you've got several rows of edges to work with.
In vertex mode, move the verticies around so that they fit the shape of the neck better. Switch to front view and pull verticies inwards so that it fits the shape of the neck from that view.
Go into Perspective view and adjust anything that doesn't look right. Cut in new edges around the jaw if you need to, to define the shape better