I don't remember where I read it, so don't quote me on this, but years ago when we rescued a foal with contracted tendons (not nearly as bad as your horse though) I found evidence that antibiotics might help. I don't remember why or how but I do know that after the little guy got a few shots of penicillin his legs DID look better. (He also had septic joints, that's why he was getting the shots.) Combined that with a good trim--his poor little feet were full of absesses, the smell was sickening--and I noticed he was improving. I did have step-by-step pictures of this but the desktop computer they were on crashed and I didn't spend the $200 to get my pictures back. :(
I gave the colt to an experienced rehabber to continue his care when I hurt my back but unfortunately he was kicked by another horse and had to be put down. We tried our best.