Man!!! what a tough hair style to draw! It's looking great! Colin it's up to you how far you want to go. You could spend hours upon hours molding.
As far as the highlights go, if you want to "pop" them I use a pen eraser cut square and run the edge down the individual strands and pull the highlight that way. I've attached a pic to show the tool. this works great for me. Give it a rip!
BTW. I did use ritmo charcoal on my pupil. worked great!
Peace!
Marc Forest.
http://www.marcforest.com