Boris Johnson will need the support of the DUP. Which he will only get on condition not to pursue May's deal.
And even if he did they wouldn't vote for it. And without them, he has no majority at all
May's deal is not coming back to parliament. And a no deal can't happen until November.
So Boris Johnson's options are new deal or no Brexit.
So Boris Johnson's only remaining option is no Brexit. But he can't do that either, so . . .
General election. Because there is nothing else. QED