Received form 1099-B for juvenile son

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!