My dog does the same thing this time of the year. When the furnace goes on she gets very dry itchy skin. Add a spoon full of sunflower oil to his food. Takes a couple of weeks to get into their system but it does help! You can also get some medicated shampoo and use it during his next bath, it helps too.
Pinworms can cause that insane butt itch, and get hatched all over. Pinworms can be picked up by people too. That or the anal sac thing. My vet showed me how to do it and coached me on it once, and that was all it took to make me pay the groomer any price she wants, just for that service.