Looks like there's a lot of dead tissue in the wound, which isn't too bad if it's just surface but that for sure needs a vet's care. They probably can't stitch things up until the necrotic tissue all dies off or gangrene will be trapped under the skin. They do this thing with wet to dry bandaging to try and keep healing tissue clean and healthy while allowing dying tissue to die off without infection spreading.
Many vets have Good Samaritan policies where they don't bill the folks who bring in a wounded stray.
Good luck and let us know how he does!
The Zombie Mama is here!