How do I do an amendment?

Can you clarify your question?  What did you forget to claim on your return?

you don't have to claim your son if he qualifies as your dependent. however, if he can be claimed as a dependent he must check that box on his return even if not claimed.  the IRS may catch the Social Security omission during its review process and send you a notice.  TurboTax says it will not have the amended return form - 1040-X - ready until 3/25. 

you can claim him/her as a qualifying child if all these tests are met
• he/she had the same principal abode as you for more than ½ the tax year. Temporary absences like for school are ignored
• if he/she is not a full-time student, he's/she’s under 19 at the end of the tax year. If a full-time student, he's/she’s under 24 at the end of the tax year.
• he/she hasn't provided over ½ his/her own support
• he/she didn't file a joint return unless there was no tax liability but merely filing jointly to facilitate refund of taxes withheld or estimates paid

or you can claim him/her as a qualifying relative if all these tests are met
• his/her gross income for 2020 was less than $4,300
• you provided over ½ his/her support
• he/she isn't a qualifying child of another taxpayer