Well, if the healing didn't take any effect on scar tissue, there would likely be a lot. Scar tissue is very stiff, and thick, making it less flexible. So if you wanted to have mounds of scar tissue like that (and it would be all the way through, muscle gets scar tissue as well), he wouldn't have the full range of motion. He may never get the full use of his arm back, but he could get most of it.
Otherwise (and even if you do the massive amounts of scar tissue, I don't think this next part would change), I would treat it as you would IRL, but instead of months of healing it's got magical healing. Which means you still have new muscle/tendon/skin growth, which means everything is going to be weaker than it once was. It's not going to be as strong until he does some physical therapy with it. Due to the building of new muscle, it'd be sore, just as if you were trying to rebuild muscle irl. Every time you used it, it would be a little sore, and if you misused it and pulled it, it would be stiff and even more sore. The same for if any bones were broken in the process of being ran through. They'll also be weaker and easier to rebreak, as will the rest of the injury. It may always remain weaker and easier to injure again. He may also have arthritis from now, even if you just rip tendons, you can still get arthritis in joint areas. So I guess that is a matter of how close it was to the actual shoulder joint.
That's if you wanted to play it more realistically. We're in a fantasy setting after all.
Otherwise (and even if you do the massive amounts of scar tissue, I don't think this next part would change), I would treat it as you would IRL, but instead of months of healing it's got magical healing. Which means you still have new muscle/tendon/skin growth, which means everything is going to be weaker than it once was. It's not going to be as strong until he does some physical therapy with it. Due to the building of new muscle, it'd be sore, just as if you were trying to rebuild muscle irl. Every time you used it, it would be a little sore, and if you misused it and pulled it, it would be stiff and even more sore. The same for if any bones were broken in the process of being ran through. They'll also be weaker and easier to rebreak, as will the rest of the injury. It may always remain weaker and easier to injure again. He may also have arthritis from now, even if you just rip tendons, you can still get arthritis in joint areas. So I guess that is a matter of how close it was to the actual shoulder joint.
That's if you wanted to play it more realistically. We're in a fantasy setting after all.
