Update:
Tomorrow is two weeks since my fluffy friend was attacked and injured. I was out of town and my parents cared for him for 4 days. Dad is a life-long show pigeon raiser who hand rears babies for show and so is used to tube feeding birds. They did amazingly well with him, my mom even giving him "physical therapy."

He is eating heartily (chick crumbles) and drinking well all on his own now. I could actually feel food in his crop that he put there himself for the first time since he was injured! His eyes were clear and alert, unlike the foggy dazed chicken I had last week. He is acting like a REAL chicken again, albeit an injured one. Even making good, normal poops. Like TEXTBOOK poops LOL.
He tries desperately to stand up but absolutely cannot put any weight on his right leg. So, he flops around, pushing himself with that good left leg. He seems to be twisted toward that right side, too, making me worry about damage to his central nervous system or something.
I sat with him in my lap, supported him with one hand and felt up and down his right leg with my free hand--his most obvious pain came when I touch the joint (wrist or knee??) between his metatarsus and tibia, according to a diagram I found. This joint I'm 'pointing' to here...
He was most comfortable with me holding him under my arm so he could lean his right (lame) side against me and gripping my hand of that arm with his
good foot. We have him in the sling with a PVC pipe wrapped in a cloth and duct tape underneath him so he can feel like he's roosting, but my sling is a stretchy, droopy disaster and I'm trying to refine that with a new, updated model. I will try to post video of him trying to stand up tomorrow evening after work.
All in all I'm pleased and amazed with the progress he made in the last 4 days under my parents' care, but I'm still very worried about him. And I will be until I can see him stand up and walk around so he can go outside and be with all his girls in the coop.