I've seen this before. The sides of a man's fingers during the summer and while experiencing work, can sweat with a heavy mixture salt on the surface. Take notice of your son's hands during sweating, wipe the hands off especially between the fingers with a black rag. If you notice salt residue on the rag as the rag dries, this could be one of the reasons for a slow recovery. Another indication of heavy sweat are salt residue on hats and shirts.
Body salt combined with sweat and body oil encourage rapid bacteria growth. That growth by the way is that locker smell that you experience in gyms and such. That bacteria could be one of the reasons for your son's slow recovery.
However, warts are common and part of the body's chemical process. They can sometimes take months and even years to correct.