He's only six months old, so there is a chance he will mature enough to dominate the older hens. There is also a chance that more dominant hen will never submit. I think it is probably more will power than pure physical strength. Gritsar said she currently has a hen that will not readily submit, but it sounds like they are able to exist without seriously injuring each other. If I remember right, her's are about a year and a half old. Her flock is also bigger than yours, 12 or 13 total? That could make a difference.
I'd suggest keeping the hens separated until they are recovered, then maybe reintroducing the less resistant hen by herself and see what happens. Then, if it goes OK, try reintroducing the more aggressive hen. What I'm thinking is that a chicken will lose their place in the pecking order if they are separated for a while. If she comeback in at the bottom of the pecking order, the flock dynamics may work themselves out. But again, they may not.
They are living things and you can never tell exactly what will happen, but I think this is what I would try. I would be prepared to get rid of one of them though. I really think this is going to be a difficult one.
