My son has an itchy rash on the heel of his foot. what is it ?

Many possibilities: There are many types and causes of skin rashes. Several types of inflammatory conditions of the skin, allergic reactions, and sometimes bacterial, viral or fungal infections can cause rashes on the skin. Insect bites can also be the cause of a rash on the skin. Have it evaluated by a professional and get the appropriate treatment.

Athletes foot: Hard to say for sure without a picture or more complete history but most common cause of an itchy rash in a young man is athletes foot which is a fungal infection of the skin. Try an over the counter spray and make sure to disinfect the shoes and bath and always keep the feet dry. If that fails follow up with his doctor or a podiatrist or dermatologist.