I've noticed teeny white spots on my sons teeth. What could it be?

See a dentist. If you have not already seen a dentist please take your child to have these white spots assessed. Some times white spots are a sign of cavities starting.
Something/nothing. White spots on teeth are generally one of two things- either an area in the enamel that the calcium distribution is a different pattern than the rest of the tooth (harmless) or it can be the beginning stage of a cavity when the tooth is still intact but the calcium has been leached out by acids from the diet, such as sodas, or by acids secreted by bacteria that are in plaque. See a dentist to tell.
Not to worry. Enamel hypocalcification is a defect of tooth enamel in which normal amounts of enamel are produced but are under mineralized. An analogy to them are like the white spots that you see on your finger nails. If they are not aesthetically pleasing they can be made to be less noticeable. There was an article once written 101 reasons why, sometimes trauma when the tooth is developing or a fever...