This is conservatives were talking about maybe he's supposed to fail in his negotiations to try and tip the balance to an out vote. Thus allowing them to enact whatever diabolical plans they have without EU interference.
I think Cameron's naturally pretty pro-EU, as are his inner circle, but he's allowed himself to be outflanked by the Tory's Euroskeptic wing repeatedly as he lacks the steel to stand up to them.
Leaving the EU would let them slash worker's rights but the really nasty stuff is subject the ECHR and leaving that is a separate issue to leaving the EU and I don't see much prospect of that happening despite the sabre rattling. There's still too many sensible Conservatives left to push it through on his narrow majority.