To what extent did Boxer learn the alphabet?

From the text:

Boxer could not get beyond the letter D. He would trace out A, B, C, D, in the dust with his great hoof, and then would stand staring at the letters with his ears back, sometimes shaking his forelock, trying with all his might to remember what came next and never succeeding. On several occasions, indeed, he did learn E, F, G, H, but by the time he knew them, it was always discovered that he had forgotten A, B, C, and D. Finally he decided to be content with the first four letters, and used to write them out once or twice every day to refresh his memory.