Below is from IRS Publication 501. Your son is over 24 and only capital gains on a 1099-Div would be eligible; no capital losses (or for that matter gains on a 1099-B).
Election to report child's unearned income on parent's return.
You may be able to include your child's interest and dividend income on your tax return. If you do this, your child won't have to file a return. To make this election, all of the following conditions must be met.
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Your child was under age 19 (or under age 24 if a student) at the end of 2020. (A child born on January 1, 2002, is considered to be age 19 at the end of 2020; you can't make the election for this child unless the child was a student. Similarly, a child born on January 1, 1997, is considered to be age 24 at the end of 2020; you can't make the election for this child.)
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Your child had gross income only from interest and dividends (including capital gain distributions and Alaska Permanent Fund dividends).
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The interest and dividend income was less than $11,000.
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Your child is required to file a return for 2020 unless you make this election.
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Your child doesn't file a joint return for 2020.
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No estimated tax payment was made for 2020 and no 2019 overpayment was applied to 2020 under your child's name and social security number.
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No federal income tax was withheld from your child's income under the backup withholding rules.
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You are the parent whose return must be used when making the election to report your child's unearned income
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