Bumble foot can eventually kill them from the infection spreading into the bones. Catch him at night to treat him...the sooner the better. The longer you let it go the harder it is to get rid of.When I read of someone having this problem, I always want to jump in and tell my story. I had a chicken doing what yours is doing...couldn't use her legs. I went to the store and got a bottle of B12 complex capsules. I put her in a cage by herself (still outside) and opened the capsules and put the powder in and on everything that went into her mouth...all water, all food. I paid no attention to how much she was getting...I just made sure she had it all the time.
In about 2 weeks, she suddenly started standing. Wobbly at first, and I kept her in the cage for another 3 days or so. Then put her out with the others. Of course they pecked at her, and she did fight back. Soon they ignored her, and now she has the biggest and strongest legs of any chicken I have. Also she started crowing and I had to rename her...from Barbara to Bob.
He has developed pads on the bottom of his feet..with dark places in the center. Doesn't look like infection, and it doesn't seem to bother him, but I'm so afraid it is bumblefoot. He is too big for me to handle by myself, so I'm just ignoring it and what? Hope it goes away? I guess I have to face it one of these days.
Try the B12 complex capsules. It can't hurt, and might work like it did for me.
Good luck!