Please see the following links:
Foot conditions
Bumble foot
We can only provide references and opinions, not medical advice. Personally, I have a couple of boars who get some dry skin/spurs on their feet that I have to trim every few months. Note that in your image the foot pad is a little reddish and the "lump" you are describing is slightly discolored. Also consider the bedding your boys are on - is it possible they're spending a prolonged amount of time on urine soaked bedding? That can cause some irritation, possibly an infection. Can't know for certain without consulting a professional.
As always, to be safe, worth a vet visit