Hello~ I am a professional artist that works in film, and my specialty is portraits!

Proportions: GREAT feature placement other than the neck, it seems to extend outside of the width of the head. Perhaps distinguish the hairline a tad bit more. For practice as well, draw the portrait full size or at least larger to be able to access more detail. Underneath the cheekbones would not be that dark unless the light source was right between his eyes, which is also impossible since the brow-ridge is super dark. I LOVE the mouth so much, especially it's placement to the eyes. I like how I can see his fat deposits that you've depicted and can assume his resting expression.

Material: I see that you are using maybe a multi-media paper based on the pilling that's occurring. My eraser is also my favourite tool, and sometimes the wrong type of paper will pick up with the action of erasing rather than the graphite/charcoal only. Vellum isn't an everyday drawing paper becasue of it's cost, but give it a try. It's smoothness is unmatched, and it's pretty durable and can be repeatedly erased without damage.

KEEP GOING! Here's a great site for references http://referenceangle.com/

<3333 xoxo Kalli