He should NOT claim exempt on his W-4, that is only for legal exemptions like employees of the United Nations who pay tax to their home country instead of the US. He should refile a W-4 as "single."
As long as he does not have more than $2000 of investment income, his income will not be taxed until he earns more than $15,000 for 2025. However, the whole point of the W-4 system is that it is self-adjusting. If his weekly income is less than $288 (which is $15,000 divided by 52 weeks) then he should not have federal tax withheld. If his income is more than that in some weeks, there will be some tax withholding, but that is correct since it suggests his overall income for the year could be higher than you guessed and he will owe income tax. Just claim single on the W-4, and let the system work the way it is designed.
Also, state income tax may have different thresholds. He should either file a separate state withholding for claiming single, or if he files a W-4 only, most employers will use that same designation (single) for the state withholding. Claiming exempt may also cut off state withholding, which could be a problem if the taxable threshold is lower than the federal threshold.
