If the distribution from the 401(k) account occurred in 2024 then he should be receiving a Form 1099-R from the plan administrator. Those are usually mailed out in January or early February. The amount withdrawn is entered on your federal tax return as ordinary income and taxed at your current tax return.
If he was under the age of 59 1/2 or under the age of 55 when he separated from the company, the amount withdrawn in subject to a 10% early withdrawal penalty. This is assess as a tax lability on the federal tax return.
