Hi reggiejackson44,
Please refer to IRS Pub 501, page 11, "Overview of the Rules for Claiming a Dependent.
https://www.irs.gov/pub/irs-pdf/p501.pdf
Since your son is 28 years old, he can be a "qualifying child" if permanently and totally disabled, and did not provide more than half of his own support for the year. If no, please refer to "Tests to be a qualifying relative".
As for the timing for filing an amendment, "Generally, for a credit or refund, you must file Form 1040-X
within 3 years (including extensions) after the date you filed your
original return or within 2 years after the date you paid the tax,
whichever is later."
Page 3 of https://www.irs.gov/pub/irs-pdf/i1040x.pdf
Hope this helps. Thank you.
Kochu Kitty
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