In general, a child always files a tax return for their own income, even if they are also claimed as a dependent, and having income usually does not disqualify a child from being a dependent.
In this case, he is required to file if his income is from self-employment. He must report his gross income, can deduct ordinary and necessary business expenses, and will pay about 15% self-employment tax (which is a version of social security and medicare withholding for self-employed persons). He won't pay income tax since his income is less than the standard deduction.
If his income is from a regular job with a W-2, he is not required to file at that dollar amount, unless he had federal or state income taxes withheld and wants a refund of those tax withholdings.
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