Hi there, i had a similar situation, but the opposite way round, my ex hubby and I get on very well and he is a contractor and had to move overseas for his business. I think the key is the fact you both get along for the sake of your son. Because without that a child struggles with loss, if your son (like my son who was 8 when this happened) knows that deep down he knows it won't be the last time he'll see his Dad he should be ok.
Keep talking regularly over Skype/Webcam/Phone calls etc...and i can tell you know, the best thing my son recieved was every other month, his Dad would send him a recorded message through the post (cause kids *love* getting mail from their away parents) and now he's 15, and his Dad has returned from overseas, he still gets the little dvd out now and again to watch it.
It won't be easy, i know, but if your son can see that moving to the USA will mean a better life for him and you, and your ex is willing to move mountains to keep up 'across the miles' contact, you should find the whole experience worthwhile.
Write a good ole list of pro's and cons, show your son (if he's old enough to understand) and don't forget moving to foreign climes often gives children a wonderful opportunity to experience other cultures.
Good luck! :0)