My 1 year old son woke up with a red mark, looks kind of like a pimple, then the next day it spread to 3 of them. what is this?

Lots of things: This may be something as trivial as a house spider or bedbug bite to an initial infection with herpes simplex (if someone who gets cold sores kissed him there) to a serious bacterial infection. Marked pain, spreading redness, or marked swelling should prompt a physician office visit.

Maybe skin infection: Could be a skin infection, which often can look like a small pimple, and it sounds like it may be spreading. Consider a topical antibiotic ointment like neosporin, but see your doctor if it continues as it could require a stronger topical ointment or even oral antibiotics.