The exposed bone part is what is most concerning to me - flesh wounds (even large ones) can heal well if kept clean and the animal, overall, is healthy. I've used Vetericyn with excellent results on a large wound on a chicken...it's a wound healing spray, available at most feed stores.
As @chickenmomma14 suggested, this injury would best be handled by an experienced vet. If that's not an option, keeping the wound clean and keeping your roo (and others, if applicable) from pecking at it are two key objectives in managing it yourself, IMO. I would use a non-adherent dressing like a Telfa pad on the wound, secured with an elastic bandage like Vetrap. The wound will need to be checked frequently to ensure it's healing properly (and no infection). Serious injuries like this will take time to heal - it may take months. Good luck!