If we voted to stay In Cameron ( who wants to stay in ) wouldn't need to negotiate at all.
He knows from the polls a vote now would almost certainly be Out but he also knows he will never get the treaty changes we want, and without them ,he would then be forced to honour the Out vote.
Quite simply he dare not have a vote now but hopes to con us long enough
to get himself re-elected. He will then try for a // LG , cosmetically better euro deal..... //
He could start negotiating now but he knows that is a lost cause.
He has one advantage , he knows the Labour Party and the Libs will always bow down to whatever the EU inflicts on us. So the Tory faithful would normally stay with him but not next time , they have UKIP.