I don’t believe so, unless he is a US resident. Say, with a Green Card, but not yet a citizen.
OK, Let’s make it more detailed ( but it is NOT theoretical because I know of such people ).
Mr. Wong is worth $20 million.
He deposited $10 Million of that in a Swiss Bank Account in 2010.
He applies for a Green card to the USA and was accepted after a few years.
He brings the remaining $10 Million to the USA in 2015 after migrating to the US as a permanent resident.
He becomes a US Citizen in 2020.
Between 2010 and 2020, His Swiss Deposits earns hundreds of thousands in interest, after 2020, his Swiss Bank account also earns interest.
Question: Are the interest from his Swiss Bank account during the time he became a US Permanent resident in 2015 ( money he made before he came to the US and never brought to the USA ), TAXABLE by the IRS?