Are you sure the reason that he is scratching is from dry skin? It might be allergies. Pebbles scratched all the time until I changed her food and started adding Coconut Oil to her diet. She now has totally stopped scratching. I believe it was a chicken and grain allergy. What are you feeding?
I have all of mine a bath once a week and believe that it is pretty standard of most members on YT to bath anywhere from weekly to every other week. If I wait too much longer their hair looks just horrible.