Yes. Check his back, sides, under his wings, his neck, etc. to see if there's any wounds or punctures. Look under the feathers really well they can hide a lot of things.When you say an external injury -- do you mean like a wound? What should I be looking for exactly? Thank you.
He had a Hawk on his back - he lost feathers - he may not have any wounds, but hawks rip with their beak and talons (talons are razor sharp and very impressive).
The comb doesn't look very good, but generally Frostbite alone on the comb should not be causing such a depressed state, so it would be good to see if you can narrow down what else may be going on.
Poop is green likely due to not eating well. I would still try to encourage him to eat his normal feed, but things like scrambled egg, a bit of tuna, canned cat or dog food, etc. can be given to add interest. Hydration is always most important, so do the best you can to get him drinking.