Correspondence? In Kenya? If they had no higher education after high school, who was going to admininster correspondence courses?
He apparently worked as a clerk for Elizabeth Mooney and the Laubach Literacy Society.
He would have needed to find a private, full time tutor.
Frankly, I have no idea how he achieved the credentials he claimed to have when he was interviewed in Hawaii.
From your link in post #10933; he did take a correspondence course:
Seniors life began to change when he encountered two American women missionary teachers. They helped him to sign up for a correspondence course leading to a secondary school certificate. He took the equivalency test at the US Embassy, and passed. He then applied to numerous universities in the United States, and in 1958, with the help of Tom Mboya, won a scholarship at the University of Hawaii.
More on Senior’s time as a student at Maseno School here.