If this one doesn't make it, I highly recommend a necropsy. I had one done recently at the state lab here for a roo that died. I tried everything under the sun to treat him, only to find out he had Marek's without the typical Marek's symptoms. I had used two wormers on him but come to find out that he had no evidence whatsoever of worms. I had started him on Tylan, but not soon enough. It wouldn't have made a difference anyway. I spent a fortune, only to be grasping at straws the entire time. My state lab charged $20 for the necropsy and it gave me so much information. UPS charged me $12 for ground shipping, but said he'd be there the next day. The lab is only a little over an hour away from me. If I wanted "Next Day" with a guarantee to be there by 10am it would've cost me $70. I made the mistake though of putting dry ice in the styrofoam cooler I shipped him in. He was partially frozen by the time they got him and it interfered somewhat with the testing. I recommend ice packs. In my opinion, it was money well spent because I have the answers I was desperately looking for while treating him. There was nothing I could have done.
Sorry you're going through this. I hope your leghorn pulls through this.