Again - universities know not to expect foreign schools to navigate U.S. systems. If they are willing to use e-docs, then the better. But sending them to the college by mail should be just fine.
Here an example what to look for (it is not a college that your son would apply to, but the concept is the same):
https://barnard.edu/admissions/internationalstudents
And they’ll likely have a department/person dedicated to international students where you can clarify any concerns, e.g.,
https://barnard.edu/international-students