Sorry for being absent; I haven't been on since the day I posted, b/c this time of year is crazy for us and I'm either working or going or just trying to get chores done.
I read through about your trials with the leg and without being able to put my hands on yer roo, I would ask if he seems to have normal range of motion when no weight is on it (compare to good leg, or to that of an uninjured chicken.) If so, how much does he seem able to move it in response to a stimulus (still without weight bearing..) toes, ankle, hock, knee, and where the thigh meets the pelvis. If he has pain when you try to move these joints or they won't move without forcing (which you probably don't want to do unless you are very slow and careful and he doesn't protest,) that indicates either an injury or contracture. Contractures are where the joints freeze up from lack of motion; it's actually a shortening of the connective tissue/muscles from inactivity. You may need to manipulate these gently some every day if he can't (passive range of motion) or help him if he can move them some but isn't strong enough (active assistive range of motion.)
What I've done on several occasions for injured legs, but particularly in the case of Walter, my current alpha, whom I had to rehab twice -- he had an injury that I think was caused by another roo attacking him; he presented like the tendon on the back of his leg was damaged, but not severed. If you have a severed tendon, nothing but surgery to reconnect or divine intervention will fix it as far as I know. But I believe any degree of tearing (if tissue is still connected) has promise for at least some healing with the right combination of rest/nutrition/PT/love/etc... We were told in school that stretching of ligaments is a permanent situation, but I do not believe this at all.. if tendons can contract from lack of activity, I would certainly think this could happen with an overstretch as well.. another subject which probably doesn't concern Henry, but anyhow..
With Walter I stood him up holding the hock joint straight with my hand (this is the part that looks like a backward knee, and for him, the site of injury.) None of the other joints seemed compromised, so I made a splint of sticks and duct tape (I would wrap with gauze prior to taping next time, b/c it really pulls the feathers when you remove it.) It wasn't tight enough to keep the joint locked into extension (straight) but it kept it straight enough that he could actually prop on it to stand for short periods at first. I also did exercises with him holding up part of his weight and allowing him to gradually build his atrophied muscles back up to being able to bear weight on that side. He started off wobbly and had a hard time because now he couldn't completely bend the joint (which was necessary in that case for healing, but you don't want to keep it that way too long, because as it heals it needs to stretch a little.) The splint had enough play to give a small range of motion and still do it's job propping and protecting. As he pecked and wrestled with it over the next couple weeks or so, the tape naturally loosened some, but long story short, he began walking within a week with a terrible limp. After a month he could limp pretty much all over the place and I think I took the splint completely off around 6 weeks (redid it a cpl times.) It took him about 4-6 months to get to where he could run (still with a limp,) but after almost a year, he has barely a limp at all, and possibly none unless you are looking for it.
All this takes considerable time, which you seem to be spending anyway, so there's no reason you couldn't try it and see if it works. Diagnosing the injury is the tricky part, so pay close attention to yer guy's reactions. Discomfort is expected an often necessary, especially where contractures start to develop, so if he flinches, just go really slow. If he makes a noise or vocal protest, that's probably too much. You don't want to injure him further.
Thanks for keeping us posted and for your commitment to seeing him through this. So far, I haven't seen anything in your posts that leads me to believe he can't make a full recovery, so that's still what I'm hoping for. Do you have any pictures?