As harsh as it sounds, I think there is little hope for this little guy, and you should cull. There is a large defect in his body wall for that much material to come out. Egg yolk is a perfect growth media, controlling infection as the sac is ruptured is near to impossible. If it gets stuck on anything, he may pull more of himself out- the yolk sac is directly attached to a portion of the GI tract. His best chance is being well cleaned, the external and contaminated material removed, any GI tract out- put back in, the defect closed, and he is put on systemic antibiotics. If he is left with that thing dangling out and confined to a damp paper towel, he may live for a day or so, but beyond that- it would take a miracle. So again, as sad as it is- I would cull now to prevent his continued pain and suffering.
If more hatch like this (or very few hatch), there may be an incubator problem with temps or humidity.