My 2.5 m old baby has small pimples all over his face and even on his arms. why is that?

Baby acne: Baby acne is very similar in origin and distribution as that of a teenager.The hormones that enabled mom to carry through pregnancy cross into baby & after a time away from the moisturizing waters of the womb will cause baby acne. The face & upper chest & back are most often involved along with the shoulders . All the lesions seem to be a similar age & gradually fade over time.