Depending on how he tracks his income, he may not need to do anything with the 1099-K as it would create double reporting of the income.
Ex. He is a plumber and tracks all of his income and expenses using QuickBooks Self-Employed. He would report ALL of the income and expenses that he has made or spent throughout the year no matter the payment method. This income would include the income he received using Square, Cash and Checks. So he will just use the numbers from QuickBooks to report ALL income and not enter the 1099-K. If he would then enter the 1099-K, this part of his income would end up being double reported and he would end up paying more in taxes than he should.
As long as the 1099-K income is included, it is fine. He will just keep the 1099-K for his records.
If he did track the income separately then he can report it in the business income section under income (This would not be common for most business owners)
How to enter a 1099-K
Form 1099-K