Daniel & the brachial plexus

I believe this question has been raised before, but I still get email about it so I thought I'd put it front and center:

I have just finished The endless Forest and I'm still puzzled by the nature of Daniel's injury. For a long time I thought maybe it was dislocated but now I think not. What is actually wrong with his arm i.e how would it be described in modern terms?

When Daniel was shot and fell out of the tree, there was severe damage to the bundle of nerves in the shoulder (the brachial p...