I can only imagine how painful it is for you to be separated from your son and your grandchildren in this way. But you are correct: Your daughter-in-law is not solely at fault. Your son is allowing this situation to exist. Although it is certainly appropriate for him to put his own family first in many ways, there is no reason for him to do so at the expense of your relationship with him and his children — and his wife.
His wife is apparently asking him to choose between her and his family of origin. This is generally a disastrous demand. No one should have to choose between the people he loves and the people who love him. Such a separation often eventually erodes the marriage.
You have ended up putting him in the same bind his wife did. Telling him to leave you alone until he can behave differently only deprives you of seeing him.
I suggest that you bide your time. Love him as best you can. Let him know how much you miss seeing him and his family. Repeat that you are prepared to love his wife as well — and mean it. As Michelle Obama said in her convention speech, “When someone goes low, go high.” You will feel better about yourself and you leave an opening for your son to reconnect at some point.
I wish you well.