Immigration Law Questions? Ask an Immigration Lawyer.
Your son would be considered to be a child of your wife for immigration purposes. There is not an independent stepchild option on the form.
Your son could apply at any US consulate in Mexico. Ciudad Juarez would probably not be a good option at all considering the recent violence that is occurring in that city.
Does your son have family that he can stay with in Mexico while his application is being processed?
You can expect up to a year for the entire process. My recommendation would be Merida because of its relative close proximity to Cancun. In general, U.S. consulates in Mexico that are close to popular U.S. tourist locations are the safest and Merida from most accounts seems to be relatively safe. The contact information for the U.S. Consulate in Merida is below.
You are welcome.