The IRS has based the amount of the third stimulus on your 2019 tax return as they have not yet processed your 2020 tax return. And from the 2019 tax return, all four people were eligible for the third stimulus and you received $5,600 ($1,400 x 4).
You can keep the stimulus for your now married son and do not have to return it even if he is no longer your dependent in 2020.
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