I had that happen before. Keep him separated, the others will peck him to bleed to death. I put antibiotic ointment on it, and HOPE it closes up on it own. Mixed results... some make it, some don't. Good luck!
I've never used that, Neomicyn is what I've got. Just get something on it quick. He probably got the cord tangled up in his legs and kicked around while hatching, and pulled it out. That means he's a fighter. Just looked up your stuff- similar- should be ok