Your son is the child. Form 8615 is for tax on his own unearned income, not his child's unearned income. (The "Tax for Certain Children Who Have Unearned Income" is commonly called "kiddie tax.")
Does your son have any income other than a W-2 and a 1099-NEC? If not, he has no unearned income, so you shouldn't even be in the Child's Income section. If he did have other income, what are the types and amounts? Unearned income is basically any income other than from working. Some examples are interest, dividends, capital gains (including capital gain distributions), and taxable scholarship income.
It's possible that you entered the 1099-NEC as unearned income instead of as self-employment income. If so, that's probably not correct. What is the 1099-NEC income from, and what is the amount?
If you did not have any capital gains or losses, you would not have Schedule D in your tax return. In that case, leave all of the Schedule D boxes blank. However, if you have an amount on Form 1040 line 7 and the box next to that amount is checked, that is capital gain distributions from Form 1099-DIV box 2a. Enter that amount in the box labeled "Schedule D, Line 15 OR Qualified Div/Cap Gain Wksht Line 3."
