First of all, your son being under 21 years old does not mean he will be able to immediately get his Green-Card. Since your are a resident and not a U.S. Citizen, I would have to know how you got your Green Card and who petitioned you. If it were a petition that allows derivatives to follow-to-join, then you must hurry and get the petition in for your son. If not, and you must petition him the normal way as a resident, that would take years.
Brian D. Lerner
Attorney at Law