And, since he's actually incapable of doing that, resigning.
By his own admission, a psychological transformation is not going to occur. PM: I will not undergo psychological transformation after poll defeat https://www.bbc.co.uk/news/uk-politics-61934851
But unfortunately that also means that he doesn't have - and will never have - the self-awareness to resign. He'll have to be prised out with a crow bar.
In some respects, it would be lovely if he does stay on until the next GE and then has to formally give up 10 Downing Street to the Labour leader - having destroyed the Conservative Party through his hubris. 
But unfortunately, he's destroying the fabric of the UK (both economically and culturally) in the process 