I don't think he's on to Walt. The Gale thing threw him off. He will be on to Jesse now as the DEA starts looking at Gus...they'll see instances where Gus and Jesse met (probably the hospital).
I also don't think he's on to Walt. Kind of a blind spot there, he doesn't see Walt as the kind of "man" that could be involved in this.I think Jesse will end up leading him to Walt. Also wonder if they will try to redeem Jesse, maybe him struggling to go straight for real. Throw in the poison and such, and it could be quite a struggle for Jesse to walk away without his revenge, and add in the stress of Hank following him or bringing him in for questioning, etc. Dunno, just speculation while looking forward to it.