Can you fill a large can with pleasantly hot water and soak his foot 2 or 3 times a day? Chickens have long skinny toes/feet and the heat will probably penetrate enough. Since he's had it some time, it will be filled with solid pus...avian pus is a solid, not a liquid. Solid pus may never drain and may need to be resorbed...which takes months. There may be scar tissue which will produce stiffness. If it hasn't progressed up his leg and he's had it a while, his immune system is probably enough to deal with it without opening it up. if the callus is lumpy and has black areas, you could get some footrot stuff at the feed mill and paint that on once a day...formaldehyde diluted one to four with water used to be the drug of choice, but now they sell a copper based preparation that is less toxic...see what you can get.