Your son is a US citizen if born outside the US to a US citizen. You need to register his birth with the US Embassy in Brazil. You need to find out from the Embassy what is involved in getting the birth registered and whether you need the consent of the child's father to do so. Most likely to get the US passport, you will need the father's permission or consent. I'm not sure whether just to register the birth you need that as well.
Answered on Apr 17th, 2013 at 9:45 PM