My son is 12 and he got small red bumps behind his ears, neck, arms and back. What could that be?

Visit the MD. The best advice is for you to visit your doctor. If your son has red bumps on his face and neck that spread downward and has a fever, you should seek immediate medical attention. If he does not have a fever, the differential diagnosis is quite large and he should visit a doctor for a full exam for diagnosis. When exactly did the rash start? Your MD needs his medical history and vital signs.