ITIN for people who can't get SS number and sometime married people whose spouse and children are in back home apply to file taxes.
It is confusing as you mentioned that your son has US passport but do not have SS number (New rule needs SS number who applies for US passport to report IRS).
If your son has SS number then no need to apply for ITIN, only your spouse needs to apply.
Even living abroad, your son with social security number would have used it during tax filing.