Hey Coyote. I'm so sorry to hear what your son is going through. I always feel awkward towards children disease, it is so unfair! I always felt grateful I became sick at 18 and not sooner.
You're facing my worst nightmare, I am deeply touched. I hope your son recovers fast after the chemo, and he will forget the worst of it with time.
About the custom you want to make: once you've filled the holes on the clicky head (milliput works nicely for this), you might want to use Citadel's Elf flesh, mixed with some white, yellow and red to find the perfect match for the skin colour (assuming you want to make a caucasian).
Best,
Arnaud