Can I claim my son if it my claim already went through

No. If you e-filed your return, you won't be add your son. It's like putting a letter in the mailbox. You can't retrieve it. Click here for the requirements to be claimed as a dependent. 

• You'll have to wait until the IRS accepts or rejects your return.
• The IRS has announced that they will begin accepting and processing 2020 individual tax returns on February 12, 2021.

If your return gets rejected, yes, you'll be able to add your son and submit the corrected return.

If your return gets accepted, no, you'll have to amend your income tax return.

At this time, you're not able to amend a 2020 income tax return. As of today, it's estimated that the IRS will finalize the 1040X form on 2/18/2021. However check the forms availability to see any updates to that date.

Note: Generally, Form 1040-X,  Amended U.S. Individual Income Tax Return, must be filed within three years of the date the original return was filed, or within two years of the date the tax was paid, whichever is later.

•  You can check the status of your Form 1040-X, Amended U.S. Individual Income Tax Return using the Where's My Amended Return? online tool or by calling the toll-free telephone number 866-464-2050 three weeks after you file your amended return.