L.D.
Even though it is on his feet, I recommend using a dermatologist since this is skin issue. Other than warts or molluskum (sp), the other thing to consider could be 'swimmers itch'. If you have been to the beach or water, and he walked around barefoot at low tide especially, he could have caught the parasite that causes it. While it is typically an itch, I had it once and it was very painful. Even though my whole family was at the beach that day, I was the only one that got it. Something to consider. Good luck.