My dad has had an itchy rash on his arms & legs for several months .. he got a cortizone (hydrocortisone) shot, it cleared up & then came back. what could cause this he scratches till there are sores.

Can only guess.: Without looking at the rash or a having a good description, the best i can tell you is that it responds to steroids.

Eczema or contact : Dermatitis are the two most common causes of localized itchy rash. But dr. Fisher is right: a picture is worth a 1000 words. If he wants a diagnosis along with his cortisone shot see a dermatologist or an allergist. The photo is of contact dermatitis.