You're right. They will peck at it and he will die. Do you have a wire dog crate, or a covered kitty litter box (without litter) or even a large cardboard box or some chicken wire or something? The chick can go out with the others if he has his own space where they can't get to him. He would need food, water, and grit, and for the others not to be able to reach him. Let us know what you have, and we can probably help you figure something out.
Neosporin (without pain relief) or Vetericyn are good to put on a wound like that. It will take time to heal, but your bird may recover, depending on how injured he is besides the wound we see. May need to feed raw egg yolk, moistened feed with nutri-drench or some electrolytes. Just keep water in him and keep him fed with healthy food. Cooked and raw egg yolk is good (but don't leave it out more than 4 hrs so it doesn't spoil).