During an accident as a child
This small boy broke his arm and was then exposed to x-rays
These rays revealed that the boy was in fact, part-man, part-skeleton

What?

A skeleton man

Waitwaitwait, sorry, 'part-man part-skeleton'?

Yes, just as the centaur is part-man part-horse
So too is he, part-man part-skeleton

Right and which part is the skeleton part?

The skeleton... part.