there's time enough yet for him [to learn his letters and get pokey].
Most of the comments here refer to pokey in the contemporary stereotypical sense of being slow or dawdling. But there may be another, entirely different meaning to the word (depending on context), which is after all derived from the root, poke (according to Webster's dictionary): to push, prod, or jab. That particular sense of the word is clearly in agreement with Mr. Lincoln's intended context, to learn (or study).
Scholarship involves a lot of study, which involves a lot of poking, in the sense of pushing or prodding, and sticking one's nose into -- books, ideas, etc. That must be what Mr. Lincoln meant by his choice of wording, get pokey.
Even his use of the phrase there's time enough yet seems to suggest that the boy was already dawdling, and slow -- with regard to his studies. Mr. Lincoln was defending his son's education, or the lack thereof -- from criticism.
In that rather awkward social position of being subjected to what must have seemed like intense criticism or concern about the well-being of his son (the apple of his eye, his pride and joy) -- Mr. Lincoln used the phrase get pokey creatively: that is, with very original, unusual -- yet correct -- usage. He may have actually attempted to coin a phrase, with some success.
Many of the country folk with whom Abraham Lincoln was raised valued hard work and considered reading a lazy vocation. Yet Mr. Lincoln felt differently about it. He felt learning was a worthwhile endeavor. The man was exceptionally clever, so I wouldn't be surprised even if he was thinking of the phrase get pokey as some sort of sly pun, for his own private amusement -- and for those in his audience who might somehow 'get it'.
Nevertheless, we should keep in mind that pokey is not listed in the Webster's dictionary published in 1856, and that various slang uses of the word may only date to ca 1919 or other more recent times. By 1884, following the Civil War, Reconstruction, and deaths of Tad Lincoln and both of his parents -- pokey had mysteriously acquired the meaning of dull, stupid, or slow.
In conclusion, since there are both formal and informal meanings for the root, poke, and more than one or two completely different meanings (mostly slang) for pokey -- one is left to take into consideration Abraham Lincoln's personality and the surrounding context of his statement, in order to derive real meaning from it. We might also be compelled to speculate on how politics may have played into the strange manner in which a phrase coined by Abraham Lincoln eventually became adopted into common usage.