This letter is addrssed to my son who is 7 years old, and I am filling a ITIN for him, and it was rejected.
The letter states.
"You Indicated the reason for submitting Form w-7 is to file a valid US federal tax return. you did not attach a valid use federal tax return to your form w-7 or your name did not appear as the primary, secondary or dependent on the valid us federal tax return sub,itted, as required."

If you feel you can claim your son as dependent, complete your tax return and attach W-7 with your tax return and mail it.