Just wanted to add, once everything is all healed up and stable, with the advances in 3D printing, there are some sources that can make prosthetics for chickens. Might be an option down the road to help him not overwork or injure the remaining leg/foot. Do some searching online, or you may even know someone that can help. I would not try to use one until he's completely healed up, new tissue can be too tender for that kind of pressure.