I had something like this happen to my leghorn rooster, Foghorn around the same age. I got a little out of hand and built him a wheel chair out of a small moving dolly from harbor freight. I added a liquid vitamin B complex and nutridrench to his feed every day and also fed him scrambled eggs. It took about 4 months, but the wheel chair forced him over time to put weight on this legs. He didn't really like the chair and over time figured out how to push up with his legs to get out, it wasn't long after that he had taught himself to walk again by leaning against the side of a run. He is now walking around (he has a little limp) but is a proud rooster with his ladies. It is a long process that requires patience and I still don't know what went wrong but I hope Foghorn's story gives you some hope.
Good luck!
Amy