Not filing would be not only immoral but doubly unwise, given that his earnings were reported to the IRS on Form 1099-MISC.
Then again, I may not have all the picture.
It sounds like you're thinking only of income tax. As you suspected, he's under the threshold and does not pay income tax.
What he's paying is the self-employment tax on Schedule SE. Like I said, that's basically both halves of the Social Security tax (15.3% of his earnings). As you may know, there is no minimum threshold for exemption from the self-employment tax. For Social Security, the gov't taxes the first dollar we earn.
Nope.
Generally, if you have less than $400 of SE income you don't have to pay any SE tax. So, at $399.99 of SE income, the SE tax is zero. At $400.00, the tax will be $400 * 92.35% * 15.3%, which is $56.52.