I have had lots of experience with them, as has Matt. Usually they grow in the mouth, but the paws are the next most common spot for them to pop up.
Violet got them in her mouth, Sadie got them on her paws. In fact we removed one from Sadie's paw because the vet thought it was a histiocytoma. Turned out to be a regular ol' papilloma. So not only did she have minor surgery for no good reason, but then I had to wait for it to come back so her body could fight the virus and send it into remission.
From what I understand, the body has to realize "Oh, there's a virus thingy and it's growing warts. let's kill it!" before any action will take place. Sometimes the body is a little slow so they (the vets) will "crush" one to wake up the immune system. I'm sure a vet can step in and tidy up what I am trying to say in much more technical and correct terms. If he has had them for a while and the body hasn't figured it out, I'd take him back to the vet and see what they can do to help his immune system out to get the ball rolling.
It is contageous, especially to young dogs. I would restrict his interaction with youngsters while he has them.
Here's the one on Sadie's paw
And here's the oral pap Vi got on her tongue. She had more on the inside of her cheeks.