I need a quick refresher on some plot points from Season Five [of Breaking Bad]

For a variety of reasons, I never managed to watched the last half-season of “Breaking Bad.” I almost obsessively watched the rest of the entire series, and had every intention of getting around to watching it. I have therefore avoided reading much about it for fear of spoilers. But NOW…I’m finally watching the end of the series.

I’m up to ep. 12 (“Rabid Dog”), involving one plot point that I only half-remember. Jessie was on his way out of town with a whole new identity waiting for him, when he reached into his pocket and seemed to realize something. He then went to reach into his pocket and somehow realized something. The shot lingered on a pack of cigarettes and then Jessie marched off to put a royal smackdown on Saul. It’s clear that somehow Jessie figured out Walt had his girlfriend’s son poisoned.

I also remember there was something to do with a poisoned cigarette that I think was intended for Gus Fring, but never used. I’m not sure how Jessie connected the dots between cigarettes and the kid though. (Didn’t Walt poison the kid with flowers from his backyard garden?) Can anybody refresh my memory?

[Moderating]
A lot of shows have had five or more seasons, so your title isn’t very informative of what you’re talking about. I’ve edited it to fix that.

Oops! Thanks for that. Stupid me.

From Wikipedia for “End Times”:

From Wikipedia for “Confessions”:

So the realisation he has is that Saul’s friend Huell is capable of stealing his cigarettes (including the ricin cigarette).