If he earned more than 1/2 of his living expense, his capital gain tax rate is on his own (typically lower), in other words, not subject to kiddie tax. https://www.irs.gov/taxtopics/tc553
Long-term capital gains tax rate 2024
| Capital gains tax rate | Single (taxable income) | Married filing jointly (taxable income) |
|---|---|---|
| 0% | Up to $47,025 | Up to $94,050 |
| 15% | $47,026 to $518,900 | $94,051 to $583,750 |
| 20% | Over $518,900 | Over $583,750 |
