can you let him swim in the tub and rinse off the matted blood? that will get the matted bits cleaned out, and a good splashy cool bath seems to be how they get themselves feeling a bit more normal. When my guys are injured I rinse-spray with a chlor-hex (nolvasan) solution and then pack the wound with neosporin, then unless someone's got organs hanging out they go back with the flock because they get less stressed - BUT I haven't had fly issues. Maybe give him a day or so to scab over. Ducks are hardy and can recover from really severe injuries, I wouldn't write this guy off at all.