The short answer is to report the 1099 income as self-employment, and he will pay income tax plus both halves of social security tax (about 15%) instead of the 7.65% that would have been withheld from wages. His income is under the limit to pay income tax, but he will still pay the self-employment tax.
If you believe your son should have been classified as an employee, you (he) can file his return using form 8919 claiming code G.
https://www.irs.gov/pub/irs-pdf/f8919.pdf
In Turbotax, he would enter the 1099 but check the box for "I was an employee and this should have been on a W-2". He will pay 7.65% social security and medicare (his share that should have been withheld) but not the full 15% self-employment tax. Then, he must also complete and file (separately, not attached to the tax return) a form SS-8 asking the IRS to investigate the employer and determine if he was an employee or independent contractor under the facts and circumstances.
https://www.irs.gov/forms-pubs/about-form-ss-8
If the IRS rules he was an employee, the employer pays penalties, including refunding the social security taxes (as penalty for not withholding correctly, the employer has to pay both halves.) This kind of investigation is likely to piss off the sports club and cost them considerable time, money and effort to respond, so I don't think he would get an offer to work there again. That may be a factor in your decision, is this a relationship you want to terminate for a couple hundred dollars. (Of course, there may be others in the same position who would benefit from the same investigation.)
