Project RDX is a mod based on the IDTech3 engine. The goal is to make the game more complex, and balanced between the "ol' school" and "realistic" games.
That's not how you're supposed to reattach a head.
Nevertheless, he STILL has 100% hit points.
As a side note about the title, I'll probably introduce a new field medic class and remove (or at least, strongly reduce) regeneration, in the near-to-mid future.