Brushing dogs often helps by stimulating their skin which then produces natural oils. So taking a dog brush to his fur might help.
Also you can give our dog a fish oil capsule or liquid. Fish Oil is used for several animals for healthy skin and coats and is completely harmless. Just add it to their food.
Nordic Naturals and Animal Essentials are two companies I know of that provide it specifically for dogs. But you can use the human capsules (found in most supplement aisles) as long as you dose them right. For human capsules I was told by a vet for dogs 20 lbs and larger to give them one full pill. If they are 45+ then to give them 2 capsules.
For dogs under 20 lbs use a safety pin to make a hole in the top of a capsule then give the dog HALF of the capsule. To be sure, feel free to call your own vet. Some vets will have different dosages and your vet should know your dog.