My son received a 1099-NEC for work as a referee. He is 17 and earned just over $800 and no taxes were withheld. Does he have to file his own return?

Yes.  This work is considered self-employment income.  

Self-employed individuals are required to file a federal tax return once their net earnings from self-employment income is $400 or more.  

Here is how to report this in TurboTax Online: 

• Navigate to Federal > Wages & Income > Self-employment income and expenses > Start
• Answer "Yes" to the question "Did you have any self-employment income or expenses?"
• Give a description of the business
• Continue until the interview asks "how does your work fit into your big picture?", based on the information you provided, it's a side job and/or a business.
• Answer all the informational questions until you get to the screen  "Let's enter the income for your work."  Here, enter that you received common income- Other self-employed income, including cash and checks; then enter what you were paid.
• Once you've entered your income, type in your expenses in the proper categories, TurboTax Online will recommend popular categories, but refer to your records to ensure you are deducting all of your expenses