It's hard to pinpoint the cause of this rash without seeing it. Allergic contact dermatitis (a reaction to something that has come into contact with the skin) may occur on one side of the body, although it would be unusual at your son's age. "Unilateral laterothoracic exanthem" is a rash that occurs in response to a viral infection. It often begins on one side of the body, commonly in the armpit, on the chest, or on the arm. The rash may spread to the other side of the body but remains more prominent on the initial side. It goes away on its own but may take up to 8 or 10 weeks to do so. If your son's rash persists, have him evaluated by his pediatrician.