If you already filed your tax return and it has been accepted for processing then you have to wait until the return has been fully processed and amend it to add a dependent to your return. You have not given us any details about your son or your mom so we do not know if you can claim them.
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
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