He has to file a return if his income exceeded the filing threshold for his filing status, even if he received no W-2 or 1099-MISC.

Does he know if taxes were withheld from his paychecks?  Does he have any stubs from his paychecks?

He does not know if taxes were withheld (we assume, but we aren't remotely tax literate so we don't know concretely). What is the the filing threshold for him if he filed single? He does not have paycheck stubs as everything is online for that company.

The filing threshold for a W-2 employee under age 65 filing as Single is $12,000.  The threshold for a self-employed individual is only $400. Does he have any record of what he made during the year?

Well, good thing about that is we know he made less than that without the paperwork (it was a retail job with an hourly wage that would MAYBE reach 12000 for a full year). And he is below the age of 65. Does that mean he doesnt have to file?

Federal law requires all employers to send employees W-2 statements no matter how low their earnings or wages are.  If they legitimately told him he "did not qualify for a W-2", the $12,000 threshold doesn't apply to him.  He MUST file a return.  The IRS will consider his undocumented income to be self-employment.
This previous TT answer explains how to file such income in TurboTax:  <a rel="nofollow" target="_blank" href="https://ttlc.intuit.com/questions/3853005-as-self-employed-with-no-1099-do-i-report-as-1099misc-or-b...>

Thank you