Discolored teeth in a 2 year old since they first came in. Why?

Event forming. Discolorations in teeth that are evident at eruption usually reflect events that occurred in the months/yrs while the tooth was forming. A viral infection, exposure to certain meds (tetracycline) or Fluoride in excessive amounts.
Developmental. I gather from your question that your child is a year overdue for his 1st dental check-up, or this would have been discussed then. Discolored baby teeth usually indicative of something that altered the enamel as it was forming. You must see a dentist so problem can be diagnosed. Please go now.