Yeah, I remember that, but Jesse was told by the FBI agents that ricin wasn’t what caused Brock’s illness, so Jesse would have no reason to think that Walt used the ricin, as ricin was not involved, according to the doctors and FBI agents...
See ya’,
Ed
The hospital determined that it was Lilly of the Valley berry poisoning and told Jesse and his girlfriend. They also said that it was not uncommon for a child to be poisoned by it because it a common plant and the berries were sweet. It was at this point Jesse realized that Fring had not poisoned Brock, but it was too late because Fring was already dead and Jesse had helped Walt kill him by telling him about the nursing home.