He didn't. Jesse was actually right when he accused Walt of getting Saul and his goons to help him. Here's the scene where Jesse goes to see Saul from Ep 12:
That's when they swap out the pack of cigarettes. You can see Huell put something in his pocket.
Also in episode 8 you see Saul interacting with Andrea and Brock. He could've poisoned the kid and Andrea would never suspect a thing, especially with Jesse running up on her talking about

"it's ricin!".
So Walt masterminded it, Saul and his people pulled it off. They don't go into too much detail in season 5 about it, but that's what happened.