Fill in the tax form and it does the computation for you. Unless you are not sure he is qualified (foreign national, etc) and you need to check on it through the IRS?

The third Stimulus payout, which started in Jan 2021, has different eligibility rules than EIP#1 and #2, as to dependents. This payout was also a projection, using 2019 or 2020 tax returns, then reconciled against Actuals on the 2021 tax return. Stimulus info can be found at these links:

https://www.irs.gov/coronavirus/economic-impact-payment-information-center

https://www.taxpayeradvocate.irs.gov/covid-19-home/3rd-eip-and-2021-rrc/

And don't overlook that the topic Title isn't also the place to put all the part of your question; that would go in the reply/comment box. That way, you won't be cut off or limited in characters.