Hello and welcome. My name is Scott and I am an experienced small animal veterinary surgeon, I will be very pleased to work with you today and will try my best to answer your question to your satisfaction.
Well there is a simple answer here, dog cannot contract the human form of chicken pox since the virus is very host specific and will not affect dogs, only people.
does your dog have a skin disease?
But really you have no worries whatsoever about your dog catching it from your son ... he can pet the dog all he likes ...
I hope I have answered your question properly, can I help you further?