i've been thru this myself. ZERO chance he goes back to sports. Your body doesnt function the same way again.
I think its more a matter of him not having the ability to play more than it is a matter of danger. A fused vertebra
is actually very strong and the joint is safe. The problem is the body doesnt function as a whole the way it used to because of this.
Zero chance he plays again at his position. Manning got away with it for a few yrs because he is a QB that didnt take a big beating..
and his arm strength loss was overcome by his savvy and ability to get rid of the ball quickly.
PS--he sounds like a great kid with a great attitude