I suggest you refer to
https://www.irs.gov/help/ita/is-the-distribution-from-my-roth-account-taxable
and
https://www.irs.gov/retirement-plans/roth-iras
The latter contains a link to Publication 590-B - which addresses distributions from IRA and Roth IRA.
Also, I presume your son should receive a Form 1099-R from the financial institution which held his Roth IRA. He should enter that data in Turbotax under the Federal Taxes > Wages and Income section.
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