Received form 1099-B for juvenile son

[CuriousChristy]

My dad started a Mutual Fund for my son when he was born (15yrs ago) but cashed it out for me last year so I could buy a car. I am listed as the custodian since my son is still underage. It has his SSN listed on the form. How does this get filed and by who? Am I required to file this with my taxes? Or does my dad have to file it with his? Do I have to file a return under my son's name?

Thank you!

[AmyC]

Yes. The 1099-B lists your son's SSN and is officially his form.  Your son should file his own return. See What is the Kiddie Tax?, and  Form 8615. 

If it were interest and dividend payments, you would be able to file it with your return on Form 8814.

[CuriousChristy]

So even though I used/spent the funds, because they were officially/legally in his name, he needs to file his own return? Even though he has no other income (no job) and will be claimed as a dependant on his father's return?

[Critter-3]

Yes ... the child files his own return. 

[Mike9241]

be aware that the capital gain may be subject to the kiddie tax form 8615. 

  Who Must File
Form 8615 must be filed for any child who meets all of the following conditions.
1. The child had more than $2,500 of unearned income (this would include the capital gain, if any on the sale)
2. The child is required to file a tax return.
3. The child was under age 18 at the end of 2023,

The son's return cannot be filed until the parents' return is completed because the form requires certain information from their 1040. 

https://www.irs.gov/pub/irs-pdf/f8615.pdf