"Had you been there during the Exodus, you would have been left behind."
Is this the proper response? Do we want to further alienate an already alienated child?
In truth, however, our response to the wicked son is not a message of banishment and rejection, but one of acceptance and promise. Had he been there, we tell him, he would not have been redeemed. The Exodus from Egypt was before the revelation at Sinai, before G-d chose each and every Jewish child as His own. There, in Egypt, redemption was a matter of individual choice. Had he been there, he would still be there. But he was not there - he is here.
"Here" is after Sinai. Here, free is what we are, rather than something that we might elect or decline to be. True, we are currently in exile, but "on that day," prophesies Isaiah, "you will be gathered up one by one." When G-d will again come to redeem us, not a single Jew will be left behind.