Hard to know where to cut without a black scab to go by. Yes, it can be bumblefoot, but this is a tough call. You could soak the foot in epsom salts and give him injectible antibiotics, but they'd have to be strong ones for a staph infection, not that Terramycin stuff. They can get stuff stuck underneath the nail which causes an infection in the foot--them scratching around on the ground makes them prone to that, which I suspect in this case. My own Isaac almost lost a toe that way, but strong penicillin and debriding the dead skin fixed him up. Toe still looks weird and thickened, but infection is gone. I doubt gout because it's only one foot, but I'm no expert on that.