I won't take a stab at the cause because it could be many things, including allergies, hotspot on his leg, or something more serious.
My two had itchy, flaky skin and I started them on fish oil. It helped tremendously, especially with the dandruff.
If it is close to his eye and he is scratching at it, you may want to get him a quick check-up at the vets and get their recommendation. You don't want him to do damage to his eyeball.