Like hugz4all4 said, it could be a number of things. If it suddenly just started, unless you've changed food, I would think it's caused by something else. Have you changed laundry detergent/softener, shampoo, lotions? Are you lighting candles now that you haven't before? Different treats? And it could always just be airborne allergies.
It's about time for the Fall Allergy season to start, so it could be something in the air that's causing it. I know you can give childrens Benedryl, but call your vet to get the correct dosage. Poor baby, nothings worse then being itchy all over. I've gotten some shampoo from the vet called Relief that has helped some. Also Aveeno makes an oatmeal based bath powder that you can soak him in to help temporarily relieve the itching.