Yes, it is because of self employment taxes (social security and medicare taxes). When you get a 1099-MISC for services performed, you are responsible for paying these taxes (~15%) on the income. These taxes are separate from income taxes, so even though his taxable income for income tax liability is 0, he will still owe this amount.
When you are a W2 employee, these taxes get deducted on your payroll and are split between you and your employer. But, you are solely responsible for paying them either through estimated tax payments during the year or at the end of the year on your return when you receive a 1099-MISC.