So I've taken the Syd Sabertooth quest as far as I can go. Got the books and taught him grammar. Six lessons later the quest pointer calls me to the area behind the bar. When I click on Syd, he acts like he doesn't remember me, I taught the stupid bloke to read. We need the numbers, he says and a few other lines that have nothing to do with the quest.

This happens a certain amount of time into the Vagabonds questlines (or did for me) and I was locked out until I'd finished off things against the Syndicate.
Sadly not too long later it properly bugs out after kicking in again.

If you have initiated Revenge, Thy Name is Fury or Direct Action, he will no respond until those are complete.

Having had to rewind my game a bit due to a broken set of companions, I've finally managed to complete this quest (and sadly, the payoff isnt much).

SPOILER:
If Gaunt is having a go at Syd and you havent wiped out the syndicate, load an earlier save and ignore Syd until you've done that. When I didnt do that (earlier save) the final part of the quest bugged out.
I think in terms of plot its meant to act out after the Isle of Dogs.