So after watching S3E5 ... is Caleb a host or no?
I hadn't thought of him as a host prior to this episode. But now that we know large parts of his memory were erased I'm starting to lean more into this theory. We know Sereck takes everyone who could potentially cause a divergence and re-codes them including his brother (whom I'm assuming could not be re-coded). Caleb mentioned in a prior episode being shot in the head. It's quite possible the implant in his mouth used by the military to control or moderate his behavior and emotions might also contain nanotechnology to similarly affect repair to his brain. His memory gaps could be from brain injury or he might have had his memories intentionally manipulated. I'd say it's most likely both.
Dolores gave him a look at his file. But with the disclosure of files to people around the world we know Caleb's reaction was unusual. We know from Liam's reaction to a review of Caleb's file that Caleb's memory of some past events are inaccurate or incomplete. It seems reasonable to assume Dolores has all of the missing pieces and is using them to manipulate Caleb.
I initially thought Caleb was being set up as the guy who would convince Dolores she was wrong about humans. But now I suspect he's going to be the guy who exposes how Dolores, armed with enough information, is indistinguishable from "Rehoboam". I think we have to assume Dolores knew about all of Serac's experimentation, including the disclosure of how people were being manipulated that led to a global societal collapse, when she decided to distribute everybody’s files.