Try contacting the National Center For Missing And Exploited Children. If you're child is registered as missing, they'll draft up missing posters of the taking parent as well as the child.
Other than that...you should be able to find out the whereabouts of your child via passport records. Entry/Exit dates on the passport should give you an idea where they are.
You should also attempt to get a Yellow / Blue notice from Interpol placed on the mother and child...this way, if they fly, their passports will be flagged and you'll be notified. (you have to have a warrant though from a police department)
Try contacting your local news and getting a segment on there as well.
And in the meantime...stay focused, and gather as much evidence as you can