I know we left and re-entered the U.S. several times prior to having his green card, however we did have to get the advanced parole on what I am pretty sure was his I-94 card. All I know is if we had left the U.S. without it or his green card, he would not have been permitted to re-enter. So check it out and let us know what you find out. Better safe than sorry. Its not worth taking the chance. I know what we had to go through to get the green card and it is best not to jeopardize it.