My 2 year old son has a recurring rash on the back of his thighs and cheeks that is sometimes just a big red area. what could be causing it?

Keratosis Pilaris: Keratosis pilaris is harmless (benign). It seems to run in families. It is more common in people who have very dry skin, or who have atopic dermatitis (eczema). The condition is generally worse in winter and often clears in the summer. Symptoms include: small bumps that look like "goose bumps" on the back of the upper arms and thighs.