I am learning on this too

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