if his income weren't self-employment, $1500 would be low enough that he wouldn't have to file. But the filing threshold for self-employment is $400, see the IRS 4012 publication page A-5, chart C, #3.
Also, if he should have been an employee and the employer misclassified him, your son can report that to the Department of Labor
https://www.usa.gov/job-misclassification and submit the SS-8 form to the IRS. These might not help him, but it could improve things for people in his situation in the future.