Portrait in progress

It looks like a brilliant start to me, Zoe.  For the lights, I would use a mixture of white, raw sienna or yellow ochre and possibly a touch of vermillion or crimson.  For the hair I would experiment with various brushes and rubber paint shapers - a different texture might be interesting.  I doubt that there is much medium tone and the dark needs to be your darkest dark.  So the form needs to be described by the highlights.  There is often a blue tinge to the highlights on black hair.  If he is wearing clothes, then I would try to pick up a touch of this colour in his hair to create balance in the picture.  However, I am not an expert and you may learn from the artists who have recently posted portraits of Oti Mabuse in the gallery.