Form 8615 requirement possibly triggered by 1099-B?, but it was actually a loss, not a gain

It depends.  The IRS will go by the selling price only, and not the cost basis of the EFTs sold.  If the selling price is greater than $2,500, then your son must file to show the cost basis reduces his overall taxable income.  If the selling price is lower than $2,500 please update here.

Form 8615 must be filed for any child who meets all of the following conditions.

The child had more than $2,500 of unearned income.

The child is required to file a tax return.

The child either:

• Was under age 18 at the end of 2023,
• Was age 18 at the end of 2023 and didn’t have earned income that was more than half of the child's support, or
• Was a full-time student at least age 19 and under age 24 at the end of 2023 and didn’t have earned income that was more than half of the child's support.
• (Earned income is defined later. Support is defined below.)  
• At least one of the child's parents was alive at the end of 2023.  
• The child doesn’t file a joint return for 2023.

For these rules, the term “child” includes a legally adopted child and a stepchild. These rules apply whether or not the child is a dependent. These rules don’t apply if neither of the child’s parents were living at the end of the year.