You have not mentioned your son's age. Are you asking about this because of the child tax credit payments that will begin in July? If so, there will be a portal with the IRS to enter information for that---but it is not up yet. If he is too old to get the child tax credit then you do not "do" anything about claiming your son as a dependent until you file a 2021 tax return next year in 2022.
More information will be available soon; the details of how it will be implemented have not yet been clarified fully. There may be monthly payments going out beginning in July 2021, with some of the CTC still left as a refundable credit on your 2021 tax return next year.
https://ttlc.intuit.com/community/tax-topics/help/how-will-the-stimulus-package-impact-me/01/1393859...
https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
