Chewy is on the right track. Tips are reported as % of sales for Service Industry. For instance, if he sold $100 of food, then it would be expected that he would receive 15% tips normally on that. the 15% is multiplied times 80% of his total sales, then that number 15% of $80 is reported to the IRS. The taxes on that $80 is not withheld by the Employer because it is a cash transaction.
Your Nephew will be responsible for paying the tax on his TIP wages. Some Service people avoid reporting their income but in fact do themselves harm. For instance, getting a LOAN depends upon documented income history. If he only "made $8k" annually due to $4/hr, no Lender will grant him a loan. By declaring TIP INCOME, he raises his GROSS income despite being responsible for the taxes at the end of the year. At least he is building income history.
Hope that helps?