Why do my 2 months old have a rash on his chest, face, behind the ears? i used aveeno (oatmeal) moisturizer his face and it healed then keep coming back again.

This sounds like: it could be eczema. You should have him seen by his pediatrician or a pediatric dermatologist, to be sure and get the proper treatment. At his age, you don't want to be treating him with unnecessary or incorrect medications. If it is eczema, it could be precipitated by something in his environment, such as his formula, or if breastfed, something in your diet, or a detergent or soap.