Can you set up a small pen for him? Another thing to do is let him out an hour or 30 minutes before dusk under your stict supervision. That way, he can get some time out to free range and will be easy to catch by roosting (he might return to his coop on his own).
Same here. One of my bantam roosters had spurs so sharp that the tip felt like a needle. Another one of my roosters had such scary spurs that when I rehomed him because I was retiring him, the person getting him almost didn't take him because he was scared of the rooster's spurs. (The rooster was a good rooster who wasn't mean or anything like that.)
As far as the cockfighting issue, that's a common first thought. (I've even deal with that thought on my first rescued rooster I got in my first year of chicken keeping.) With this rooster, he doesn't appear to be a breed that would be used for such illegal activity, so I highly doubt that's the case. Another thing to consider is roosters literally get abandoned all the time (in cities, back roads, and the most common place of abandonment: farms, both private and non private, that has chickens, which may very well be why he got abandoned at your place). People accidentally get a rooster when they didn't want one, discover they can't have roosters in their area, or the rooster turns mean, and they don't know how to rightfully deal with it. In this case, the poor bird could have gotten into whatever was on him, and a possible vet bill scared the owner and they did what they "thought was best" (which clearly never is).