Yes, so long as your "ex" has joint legal and physical custody of your son, the "ex" has the legal right to have the social secuirty number of his child. I see no reason why you would refuse to give such to him. It is better to give the social secuirty number to the "ex" of your son than to pay attorney's fees fighting over a matter that he is entitled to have.Perhaps the "ex" is setting up an account for the son and needs the social security number to do so.