Call ahead first make sure your vet knows and allows non vaccinated dogs. Like i mentioned before my vet has a specific area he prefers unvaccinated dogs in. Let them know what the problem is ahead of time and it's ok to get an estimate of cost. The cost of the visit it's self and anything that they may test for.
Can I also suggest something to try first since it doesn't seam to be painful. Their are different names out there but the generic term I know it know as is "hot spot" Its the irritated skin area. I am not saying a vet exam is not needed but, in the mean time till you can get him in or convence your parents to get him into the vet this may be beneficial
Veterinary Q & A: Hot Spots - your veterinary questions answered