@AcePL: I suspect that it's the required form of ID when a US citizen presents themselves to US customs, but I would be surprised if it's legally required for other purposes. (That said, a US passport will probably be recognized more readily in the US than will an Austrian driver's license.)

"Therefore, any employer, including a non-resident employer, is required to withhold amounts on account of the income tax liability of an employee in Canada even if the employee is likely to be exempt from tax in Canada because of a tax treaty. For the employer to be relieved of their obligation to withhold, the employee would have to apply for and get an income tax waiver from the Canada Revenue Agency (CRA)."

The employer has Canadian filing obligations even if the employee is not a resident of Canada for tax purposes: "Non-resident employees providing employment services in Canada are subject to the same withholding, remitting, and reporting obligations as Canadian resident employees. ...

I think Google Voice will work with cellular data as well, so he wouldn't strictly be tethered to Wifi with that. If that's correct, he could get a local smart phone with a data plan and still make & receive calls to his US number. But I've never used the service and don't know for sure if this will work, hence this is a comment rather than an answer.

Google Voice might be a viable option, though if I understand correctly it would mean that calls to & from the US number would only be accessible via a data connection. This may or may not be viable depending on how reliably your son will be able to access WiFi or cellular data (and how much it will cost.)

Related question from same person on Law.SE.

Did you apply for a re-entry permit, a preservation of permanent residence, or a returning resident visa in conjunction with your 2014-2015 trip?

Note that if the OP needs to wait "at least 4 years from your latest trip date of a year or more", the earliest they'd be able to apply would be 4 years after 11-13-2015, or 11-14-2019. (How did you arrive at the August date?)

I suppose you could retroactively file the "past two years" of returns, even if it included time that you weren't actually present in MD, But that seems like a strange requirement.

The one you linked to in your question — here.

The MVA FAQ says that someone without US citizenship has to have two years of Maryland income tax returns in order to get a driver's license. How does that work with the requirement to change their license over within 60 days?