I'm learning myself, so take this with a grain of salt. On the pear, you've got the shape of the shading down fairly well, I think most of the issue is just working on value. You've got some dark areas which appear to be as light as the highlights and the difference between the light side and the highlights seems to extreme. It is hard though when you are trying to work with just 3 or 4 values. On the portrait it looks like you have reversed a lot of the darks and lights. The shape of the right side of the face is odd, and I think this is mostly because of the reversal. The shape is not far off since it includes his hair, but since you've made it light it looks like his jaw is oddly shaped. If that area was in shadow as in the photo I think it would look much better. Rather than shading the white shirt you could have lightly shaded the background on the left side of the head, which would then give you something to form the shoulder but leave the shirt bright. With the black hair a light shade in the background would not lose the shape of his head. I think shading the shirt is a big part of what led you to reverse the shading on the head. This exercise is very similar to the planar head, and a common difficulty encountered is trying to add detail you know is there, but is not visible due to the shadow. You seem to have done exactly this on the right eye. That eye is mostly lost in shadow on the model, but you have made it stand out quite distinctly.