It depends on filing status and if he can be claimed as a dependent on your tax return. If he filed as Single, there will be a $6,300 subtraction from his taxable income. If nobody else can claim him as a dependent on their tax return (yourself, for example) he will get another $4,000 subtraction for the Personal Exemption amount. That's. $10,300. If he earned less than that and all of the above is true, he will get a full refund of federal taxes.
Anything above that would be taxable income (assuming there are no other deductions or credits allowed). For 2015, he can have up to $9,225 of taxable income that would be at a 10% tax rate. (Taxable income is the amount after the subtractions above.)