Most commonly, I see interdigital cysts, especially with a history of allergies. They can become quite painful and infected.
However, there are other growths that can be there that are not so benign.
If there are multiple bumps, it makes me think that this would be less likely
, but it is another possibility.
Most of the time, theses cysts come from a deep skin infection that may require antibiotics. The key is getting them to stop the licking. You could try a foot soak with some betadine or other antimicrobial. I would do that every 8-12 hours for several days. If they aren't resolved, change in any way, or gets more lesions, I would have him examined.