How old is your son? Have you seen your doctor about this? I am not sure if anything can be done about your son's foot being smaller than the other. Perhaps there may be orthopedic shoes to help with his balance and walking. There are individuals born with one leg a bit longer than the other and it is a matter of having specially designed shoes or inserts for them. I think it would be best to talk to your doctor.
when can i receive an answer