It might be different with a slow maturing breed, but I don't have any first hand experience. Was he crowing and starting to chase pullets yet? It's the testosterone ramping up that tends to make cockerels get tougher more quickly than pullets.
Agree with the rest.
That green stuff was most likely the bile duct being broken. But, if you washed everything well, you should be fine.
And, yes, yes yes. Do not treat the cooking of this cockerel like a normal supermarket chicken. It will both be more flavorful (in a good way, I think)...