My son forgot to say yes to a question that someone can claim him how do we fix this so I can claim him on my taxes he already file is taxes this year

[PatriciaR]

If you can claim your son as a dependent, he can amend his tax return to show that "someone else can claim me as their dependent on their tax return" and you can mail your tax return to the IRS.  Follow these steps:

Here is a TurboTax FAQ for your son to amend his tax return.  Form 1040X isn't available yet, but should be very soon:

https://ttlc.intuit.com/replies/3288565

To prevent fraudulent returns, the IRS will only accept each dependent's SSN once in their e-file system. Because your son filed first and used his SSN, no further e-files using that SSN will be accepted. 

You will need to print, sign, and mail your tax return to the IRS.  Your tax return will be accepted if you paper file and your son amends his tax return.