Check the bottom for a scab. If there is one, look up "bumblefoot" and there should be threads with methods of treatments.
It involves soaking the feet, removing scab, digging out pus and so on, with injectable antiboitics IF it is an infection there. Regular soluble water stuff won't do anything for bumble foot IF he has it.
If no scab can be found... could be another growth, from which antibiotics wouldn't do anything.
Antibiotics ONLY kill bacteria. They kill 99.9% of the good bacteria you want in addition to the few you don't want. Due to the major antibiotic resistance issues, you want to avoid using them unless there is an infection or a high risk such as open wound or animal bite.