underneath the jaw there should be a shadow going from the ear to the middle of his chin since by shading the outer parts of his jaw a darker shade you need to make the skin underneath the back part of the jaw blend with the rest of it, the reason for this is that our eyes where made to spot out differences unfortuantly so to others the dark lines among the light parts of the skin make it obvious. Another option is to just erase the dark lines where i was talking about and smudge it because near that area is where the neck connects with the jaw.