Yes. Your grandson may have to file a tax return on the amount received from Social Security (depending on how much the grandson received). Your grandson should receive a Form SSA-1099 showing in box 3 the total social security benefits paid to you. Box 4 will show the amount of any benefits you repaid in 2021. Since your grandson is single, the amount would have to exceed $25,000. A worksheet to calculate if the amount would be taxable is in the 1040 Instructions on Page 31.
Since your grandson is under 18 years old and has unearned income, your grandson may be subject to the Kiddie Tax. This tax requires the income to be taxed at potentially a higher tax rate. Information about is the Kiddie Tax is at the following link: Kiddie Tax Information
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