Hey Julie,
Although I like the drawing, there are things that bother me that were more noticeable when comparing to the reference. The values and contrast seem off, not everywhere, just here and there.
When changing it to a very dark background, maybe some of the darks should have been made darker, along with the mids and some of the lights. You've adjusted the shirt, making it a mid instead of light, but the hair, I feel, is much too light. Above his ear is even lighter than the reference giving the hair a flatter appearance. The shadow in the cheek areas, with a little reworking should go back in space properly.
I would take it slow, gently working up the darks and mids until it fits better with such a dark background, just don't go overboard!
ps. wait until you get another opinion before going ahead with mine.
