You are doing a great job, keep it up. As for your question (other thread) about bruising or pus under the skin, I agree with
@FeatheredFriends&Horses2, I think there is only one small possible area.
Any new pictures of the wound on his back?
It's good his activity is up, and that his appetite has returned. Also very good that there is no sign of infection.
There are obviously some chickens that can heal without antibiotics, but I definitely feel that the penicillin injections are making a big difference with your rooster. Not everybody has access to or can work up the courage to do such. A lot of chickens die I think because the wound gets infected, maybe spreads to the bloodstream and causes septicemia.
So good job on everything, keep us updated of course! Hoping he pulls through.