What Causes Smattering Of Brown Dots On The Stomach In Children?

Hi my son is 20 months old and today I noticed a smattering of brown dots all over his belly. Could this be an onset of freckles or something serious? Otherwise he seems to be completely fine and normal (eating, sleeping, and urine/stools). He does not have a temp. He has proven to have sensitive skin in the past but nothing that ever looked like this. Should I consult his Dr. or a dermatologist?