He will be PM but not for long, I think.
He doesn't want to actually do the job. He isn't interested in public service, or Brexit, or anything but his own careeer.
He wants to win and to be able to say he was PM. He wants to be in the books. He wants (and feels, with that massive, massive sense of entitlement that only afairly mediocre sow's ear turned into a slightly badly-stitched silk purse thanks to Eton can muster) that he deserves to be on that list of British PMs.
Once he's done so, he will fairly quickly get himself out of it.
Deliver Brexit?! Yeah right, Boris loves dedication and hard work and all that stuff doesn't he 
In another life, he'd be doing this on the X factor or Love Island. That's the kind of person he is.
So look past Boris really to what happens next, I think. Or - consider what Boris's inability to actually deliver within a post might mean for actual Brexit. He'd certainly probably find it more attractive to take a contrary view and go completely against what he's saying now, simply because it would make for a more interesting 'storyline' for posterity.
He doesn't want to do the job of PM, he doesn't want to serve his country, he wants to go down in history - nothing else.