If it's on the bottom of his foot it's probaly a planter's wart. Those are horrible, I had one when I was pregnant and nothing worked. I never went into the doctor but should have. I kept buying over the counter freeze off, picking at it, and slicing it off with a razor blade, the problem is it has deep root and you have to get the roots to get rid of it completely and that's tricky. I have no idea but after I delivered my baby it went away and NEVER came back. I know it's contagious and I could have passed it on to other's if I walked in a locker room etc.
I would maybe go to a different doctor he should have gotten rid of it the first time.
If it is a planter wart they are extremely painful, like having a pebble in your shoe, it got to the point I started walking sideways on my foot and screwed my foot up. I tried ducktape,nailpolish,over the counter freeze off, cutting it with scissors, razor blades the darn thing lingered for months. If I have this problem ever again I'm going straight to a doctor and having them cut it out of my foot.
Silly me my pregnant butt was to embarassed to go to the doctor. Not again then though.