Sign on bonus on 1099NEC

Your son should not have received a 1099-NEC for a sign on bonus.  It should be reported on a W-2 as it is employee compensation.  

You will need to contact the employer and ask for a W-2 instead of the 1099-NEC. If they will not, then you would need report the income by selecting the following:

1. Income
2. Miscellaneous Income
3. Other Income not already reported on a Form W-2 or 1099
4. Yes, to did you receive any other wages
5. Continue through until you get to Any Other Earned Income and select Yes
6. Select Employee compensation that was not reported on a W-2
7. Continue through answering the questions.  Put a checkmark in the box that says you received a 1099-Misc
8. Select letter H- I received a form 1099-NEC that should have been submitted on a W-2

TurboTax will then include form 8919 with your return for uncollected Social Security and Medicare wages.  This will have you pay 7.65% instead of 15.3%.  

There's an alternate way of reporting the income, without waiving a red flag at the IRS about the new employer. 

Your son is not yet an employee, so the 1099-NEC is not necessarily wrong.

The IRS considers anything on a form 1099-NEC to be self employment income. Enter the 1099 income, at 1099-MISC and Other Common income / income from a 1099-NEC . TurboTax (TT) will complete Schedule C  for you and allow you to deduct any expenses associated with this income. You'll also have to pay self employment tax (social security & Medicare) on any profits greater than $432. These types of payments are frequently done this way. Your "employer" is not the only one.