@yulichoco - you did everything correctly.
for your son, did he received a 1099-MISC form? if so, THAT is why he had to pay tax. It is correct.
On a W-2, the social security tax and the medicare tax is withheld from the paycheck.
But on a 1099-MISC, it doesn't occur that way. So the IRS collects the social security tax and the medicare tax on the federal income tax return- there is no way to avoid the tax just like there is no way to avoid the SS and medicare withholdings on the W-2.
if the tax was $200, then the 1099-MISC form should have been around $1415. the formula is 1099-MISC times .9235 times .153 = self employment tax.
you MUST file a tax return if any of these are true
1) income is over $12,200
2) there is federal or state withholdings on the W-2
3) you received a 1099-MISC
and if your received a 1099-MISC form then he was required to file
Trust your accountant 🙂
