His argument is that he intends to negotiate a better deal, and he can't ask the people to approve or disapprove that deal until it's on the table; and that in any case the consequences of the Eurozone crisis will include inevitable treaty changes and changes in the relationship between Eurozone and non-Eurozone EU states, again meaning that you can't ask the people to vote on an outcome which is at present unknown.
There is some logic in this position, although as a strategy it's full of obvious holes. But he didn't have a great deal of choice - without some such compromise as this, it's doubtful he could hold his party together.
His best negotiating advantage would come if he scheduled a referendum but far enough in advance that it would give the EU time to respond, but hold their feet to the fire, since they, likewise, would not otherwise negotiate in good faith.
And he could use a “carrot and stick” approach, by giving more alternatives than an “up or down” vote, based on how the negotiations are going.
Eventually, the referendum could include several variants, as separate issues, with a minimum number of votes needed to pass any particular variation, etc.
He might even throw in some wild cards, like denationalizing the BBC with the elimination of the television tax.
And absolutely schedule it before the election, to avoid both the appearance of duplicity, and to have a chance at canceling the referendum if it becomes wildly unpopular.