My son accidentally forgot to check the box allowing us to claim him, on his own tax return. He already filed. How can I fix this?

 

After you file

You will have to mail your return this year to claim him.

He will have to amend his return.  How to amend an Online return https://ttlc.intuit.com/replies/7276596 

After you file

So, in the meantime, how do I (the parent) claim him as a dependent?  Otherwise, what I owe doubles!

After you file

You claim him on your return.  You have to print and mail your return.  You don't have to wait for him to amend his.