How may I receive a refund of fed tax withheld in 2016 on my 13 year old son's SS payments?

Fed tax was mistakenly withheld in 2016 from my son's SS payments.  I would like to receive a refund of these fed taxes withheld, as there should be no tax withheld from these SS payments.  Since I do not report this SS payment on my return, how may I get a refund of incorrectly withheld fed taxes?