The studio has designated your son as an independent contractor. This is not uncommon for instructors, personal trainers and so on. Whether this is legitimate depends on the balance of facts and circumstances regarding his working relationship. Start by reading here
https://www.irs.gov/businesses/small-businesses-self-employed/independent-contractor-self-employed-o...
If he is legitimately an independent instructor, then he is self employed and is required to pay self employment tax on net income after expenses. (His income is low enough that he won’t pay income tax, but will pay SE tax.)
If you believe the classification was made an error, he can file a form SS-8 with his tax return requesting that the IRS do an investigation and make a determination of his status. If the IRS rules that he was incorrectly classified as independent contractor, the employer will be penalized. I believe that this can be accessed in TurboTax by entering the 1099 Dash NEC and then checking the box on the next page listing special circumstances that says “I received a 1099 but I was really a regular employee.“
