The Letter 6475 that you received is the amount of the 3rd stimulus payment sent to you. You have to report receiving the stimulus payment on your tax return.
Your son is a dependent so it not eligible for the $1,400 Recovery Rebate Credit on his tax return.
Letter 6475 shows the amount YOU received for the 3rd EIP payment---the stimulus check---that went out sometime after mid-March of last year. You enter that information on your tax return in Federal Review for the recovery rebate credit. They sent $1400 per person.
If you received any of the advance child tax credit payments between July and December for your son ($250/month) then you should have Letter 6419 for that.
Federal>Deductions and Credits>You and Your Family>Child Tax Credit
The IRS is sending out letter 6419 to you. It will show the amount of advance child tax credit that you received during 2021. Enter the information from that letter carefully.
The remaining amount of CTC that you can receive will show up on line 28 of your 2021 Form 1040. Then it will calculate down to the refund you see on line 35a or the tax due on line 37.
PREVIEW 1040
https://ttlc.intuit.com/questions/1901539-how-do-i-preview-my-turbotax-online-return-before-filing
Click on Tax Tools on the left side of the screen. Click on Tools. Click on View Tax Summary. Click on Preview my 1040 on the left side of the screen.
NOTE: The CTC is indeed a “credit” that can be applied toward any tax liability that you would otherwise have to pay as “tax due” to the IRS.