What do I do for my Min Pin who appears to have dermatitis with whelps..?

There are very very few things he could pass on to other dogs and being an inside dog the chances of him having one of those issues is next to none. So no reason for him to be confined. Next it is likely a reaction to something but without knowing what you are better off leaving it alone other than the bath until you can see a vet. Such a small breed should be given medicine by a vet only. Benadryl in a dog under 5 lbs can kill.