Almost there.

After adding hair and eyes to the sculpt and playing with the existing features to get a closer match to the front and side reference pictures, I went to a crit with Mark Wickham, a university tutor. Mark told me that the likeness was definitely there and pointed out a lot of small details that could be tweaked to get a better result. He then pointed out I could get further by using the photos as references on the side, instead of laying them over the model to try to find an exact fit. So, using some 3/4 photos of Jack, I was able to pick out loads of small problems with the model I couldn't see with just the front and side views. One such problem was the cheekbones and shape of the cheeks.

Using 3/4 view photographs, I was able to really find the shape of the cheeks and cheekbones much easier, as well as correct some of the smaller features.

 The cheeks lacked a lot of flesh, making him look much skinnier than he actually is. Although for some of this "fleshing out" I found that I could use the Wax tool or, in some more extreme case the sculpt tool on a low strength, the Grab tool was sometimes necessary to make major changes. Unfortunatly this often meant distorted a lot of features I had already working on, slowing me down by making me return to parts I had already worked on. On the positive side, this often gave me the opportunity to find small details I had missed before, such as my misjudgment of how suddenly and how far the mouth sticks out and how the cheeks bulge slightly around that area, causing creases. Also, whilst using the perspective photographs as rough reference I managed to find the planes of the nose to such a point that I could un ravel the shape of it in my head and make the nose much more convincing in the process.
One more important point Mark made was that the model should still be recognisable on the lower Sub-D levels. I believe I am very close to this goal, as well as completion after perhaps a few more hours work.

I believe that the face is still recognisable at lower levels as well as the higher ones.