"But in 1827, at age fifteen, he was again forced leave school and work as an office boy. In the following year he became a freelance reporter and stenographer (using shorthand to transcribe documents) at the law courts of London"
It mentions the book David Copperfield that he wrote. Some think was mostly the author's own experience. In it he wrote about taking notes in Parliament and how to do that, " a perfect and entire command of the mystery of shorthand writing and reading, was about equal in difficulty to the mastery of six languages; and that it might perhaps be attained, by dint of perseverance, in the course of a few years." Remember that this was for court reporting or similar. So a very accurate and fast shorthand was required. Plus learning a new "language" of the vocabulary they used in various settings.
Scrolling through the above link, I found a page from the "Charles Dickens, shorthand writer" by Carlton, William J. (William John), 1886-1973 Publication date 1926
At one point, I thought it said, they weren't sure exactly which Shorthand he learned, but they are pretty sure this was the the one. It was a popular shorthand and the one his father and others knew. The 15th edition would have been out when he learned it.
From the above link, the end of the book (page 138 and around there), it states he kept a shorthand copy of his books, etc., just in case. Here is a copy of a letter he wrote in shorthand. Please note, it did say he revised the original shorthand he learned to something he could write and read.
"Gregg Shorthand had achieved the most extraordinary success ever attained by any system in the history of shorthand." John Robert Gregg, 1922