I'm not sure we can tell you for sure exactly what that is, but it looks like a papilloma or wart. These are usually a benign growth and are often caused by a virus. If Marci has been in contact with other dogs recently at the dog park, a boarding or daycare facility, etc.--these are common places to pick up the virus. If it is that, he may develop more. They typically disappear on their own over the course of a few weeks. If this doesn't sound like a possibility, that raised area may simply be a spot of inflammation or folliculitis. Monitor it for growth and it us shears a good idea for your veterinarian to check out any strange lump.