I can claim my son for 2021. Do I include the amount for the stimulus payment the he received when filing my taxes? Or only include the amount I received on my letter?

If your son received a stimulus payment last year it does not go on your tax return.    You only enter the amount that was sent to YOU---use your letter 6475.

IRS interview to help determine who can be claimed:

https://www.irs.gov/help/ita/who-can-i-claim-as-a-dependent

https://ttlc.intuit.com/questions/3113432-who-can-i-claim-as-my-dependent  

You only enter the Stimulus payment you actually got.  Then you will get the Stimulus for the dependent you are adding if you qualify and your income isn't too high.  You do not report the Stimulus payment they got or anyone else got for them.   They can keep the payment they got.   They don't have to return it.

You can really  get it now.   It is based on 2021 return.   It's legit.   Their 2020 return was just used to send out the checks fast. It's a IRS loophole.

See Question 15 here

2021 Recovery Rebate Credit — Topic E: Calculating the 2021 Recovery Rebate Credit | Internal Revenue Service