Dependent capital gains

depending on your sons tax situation, he may have to include form 8615 with his return - the kiddy tax - dependent files return or 8814 also kiddy tax but parents report the unearned income on their return

Who Must File
Form 8615 must be filed for any child who meets all of the following
conditions.
1. The child had more than $2,200 of unearned income.
2. The child is required to file a tax return.
3. The child either:
a. Was under age 18 at the end of 2021,

b. Was age 18 at the end of 2021 and didn’t have earned
income that was more than half of the child's support, or
c. Was a full-time student at least age 19 and under age 24 at
the end of 2021 and didn’t have earned income that was more than
half of the child's support.
(Earned income is defined later. Support is defined below.)
4. At least one of the child's parents was alive at the end of
2021.
5. The child doesn’t file a joint return for 2021.
For these rules, the term “child” includes a legally adopted child
and a stepchild. These rules apply whether or not the child is a
dependent. These rules don’t apply if neither of the child’s parents
were living at the end of the year

Purpose of Form
Use this form if you elect to report your child’s income on your return. If you do, your child will not have to file a return. You can make this election if your child meets all of the following conditions.
• The child was under age 19 (or under age 24 if a full-time student) at the end of 2021. “Student” is defined below.
• The child’s only income was from interest and dividends, including capital gain distributions and Alaska Permanent Fund dividends.
• The child’s gross income for 2021 was less than $11,000.
• The child is required to file a 2021 return.
• The child does not file a joint return for 2021.
• There were no estimated tax payments for the child for 2021 (including any overpayment of tax from his or her 2020 return applied to 2021 estimated tax).
• There was no federal income tax withheld from the child’s income.
You must also qualify. 

Parents who qualify to make the election. You qualify to make this election if you file Form 1040, 1040-SR, or 1040-NR and any of the following apply.
• You are filing a joint return for 2021 with the child’s other parent.
• You and the child’s other parent were married to each other but file separate returns for 2021 and you had the higher taxable income.
• You were unmarried, treated as unmarried for federal income tax purposes, or separated from the child’s other parent by a divorce or separate maintenance decree. The child must have lived with you for most of the year (you were the custodial parent). If you were the custodial parent and you remarried, you can make the election on a joint return with your new spouse. But if you and your new spouse do not file a joint return, you qualify to make the election only if you had higher taxable income than your new spouse.