Yes, it is possible. Because your son made more than $400 from self-employment, he's required to file a tax return to pay the self-employment tax (Social Security and Medicare taxes). The self-employment tax rate of 15.3% is applied to 92.35% of your son's income. If his income was $712 and he deducted $70 for uniforms, he'd have a net self-employment income of $642 and his self-employment tax would be about $91 (92.35% of $642 = $593, and 15.3% of $593 = $91).
Note: The $101 you calculated was for a net self-employment income of $712, which would be correct if he didn't have any deductible expenses.
