There are GED practice tests!
The language arts section is exactly what he spent his entire childhood education doing, modulo some confusion about what the Arctic National Wildlife Refuge is and why one might want to drill for cooking oil in it. The math he understands, but his education seems to be located a little to the left of what they want. The technologically advanced people helpfully gave him a detailed understanding of both English grammar and math notation. (They invented this numeral "zero" which seems very nifty.)
The science section involves phrases like "kilojoules of energy" and "a genotype of Bb with brown eye color" and "evaluate the frictional work." The social studies section has more familiar words, setting aside proper nouns, but there are a bunch of pictures he doesn't know how to interpret and questions like "how did the earliest settlers arrive in America?" and "where did the Maori people migrate to from Polynesia?", and even if he sees something he thinks he knows the right answer to like "what would happen if the government forbade anyone from importing goods?" it tells him he has the wrong answer.
He does at least get the one about the definition of filial piety right.