Thanks for all the comments! It's possible that someone else trapped him and provided medical attention. I hope that's the case!! I can't confirm anything since I haven't seen him since Tuesday, but I'm keeping the trap set and hoping to get him. You're right, though, lindapalm - if he's been trapped once, he's not going to come near another trap. I do have binoculars. I'll have to keep them with me when I'm outside, 'cause when I do see him, it's usually when I don't expect to.
Kilter, the TNR idea is great. I already did that when I trapped the injured cat's sibling. The second cat I caught was one I thought belonged to someone. That's the thing - if I'm not sure it's feral, I don't want to mess with someone's pet. Still...I think all roaming cats should be neutered!
I'll keep you posted when there's news. Thanks again!