Your son should give you $1400. They reviewed his return first and saw both SS numbers. Tell your son it is a federal crime for him to take your stimulus check. And if he doesn’t give it to you, you will report it. There is no tax implication as these checks are tax free.
(just between us, and perhaps your son isn’t your best buddy, but next year when you file your 2021 return you can claim it for a refund. They for sure they will be after him!!!)