"except salary received from a partnership."

"The partnership did not pay the partners through payroll."

Partners don't get paid through Payroll, unless they elected to be treated as a Corporation.

If he was not a partner in the formation documents, he would be an employee and would be paid through payroll. Two different ways a person contributes to and benefits from the business operation.

"The partnership pays all the home expenses"

It's sad that people have no idea how to determine how the business is performing, because they do not manage the money correctly. I wonder if they are prepared for what that tax filing is going to show.

"He just works at the family business, which pays all his bills."

It seems time for him to learn to be in business, then. I've seen this is called "Adulting."

"The partnership absolutely did not pay him, even giving him any draws."

Yes; that's exactly what that gets treated as. Personal expenses paid from the business = draws. That would be part of the 1065 and his K-1.

"So he doesn't need an "income" per se"

He let mommy and daddy manage it, through the business.