While GED Test is open for non-US citizens, noted that 40% of candidates will fail it, so kudos to your son for passing GED just by self-studying. Seems lots of practice is necessary for passing GED.
GED® Test Performance - Race and Ethnicity in Higher Education (equityinhighered.org)
The pass rate for all test takers, including those who did not complete all four modules, was 59.9 percent.
How difficult is it to pass the GED test in 2022? - Quora
The GED is designed so that you need to perform at the level of a 40th percentile high school graduate to pass it.
In other words, 60% of recent high school graduates will pass the test, and 40% of recent high school graduates will fail it.
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First, on the math test, there’s much more algebra, and more advanced algebra, and geometry questions that requires you to have memorized some basic formulas (to find circumference, for example).
Second, on the writing test, there’s an essay that requires you to respond to two readings on the same topic, demonstrating with evidence as to which makes the best argument (and not just which one you agree with).
And, third, many folks find taking the test more challenging because it’s on a computer, rather than a paper and pencil test. Readings are presented in small sections, about the size of a cell phone screen, which means that one must tab through to read the whole of a piece (this makes skimming much harder). Essays and the responses on the science test are also written in a small space, which I imagine makes it harder to proofread your responses. Taking official practice tests can help you understand some specific features of the test, like drag-and-drop questions, the flag for review function, and the on-screen calculator.
Perhaps maths or language arts if necessary as this may be the two more challenging subjects for some candidates.