Well firstly, it should have been a 1099-MISC or 1099-NEC. But moving on,
Your son is self-employed if this is an ongoing trade or business. Is this something he did with the intent of earning money? Does he do it on a regular basis? Does he search out work opportunities, take training courses to keep up to date on rules, and do other activities that suggest this is an ongoing compensation for work he is performing. Or was it a one-off thing, not done with a profit motive, that he probably won't be doing again.
He can be considered self-employed if the facts fit, there is no bottom age limit. If he is self-employed, he may be able to deduct ordinary and necessary expenses, such as training and uniforms. He would pay self-employment tax on the net profit, even though the amount is too low to pay income tax (unless he has other income).
