I think you are right on the headbob, but the two things that are really bugging me are the fact that his feet never leave the ground, and his back knee is almost stationary, other then on frame one.
This is made worse by the fact that his silhouette is a 3 frame walk, with only the leg shading reversed on 1, 2, 4, and 6.
I suspect changing frame 3 & 4, so that on one his back leg (from the camera) is infront, and on the other it is behind (him) would do a fair bit to make this less noticable, but you should also probably invest in a frame where one of his feet is off the ground so he isnt just shuffling around.