He will find a way to come back. If he waits long enough the Tories will be so electorally weakened by all of this that I worry that they could welcome him back as some kind of populist saviour after the next GE.
I just can't see Boris going for that tbh. He'd like to be PM again, yes, so if he could get the gig again before the next GE he'd take it.
But after that, the picture is going to look very different. The Tories currently look like they'll get many fewer seats than they'd need for a comfortable, helpful period of regrouping in opposition with the realistic hope of being back in a Parliament or two. Could easily be sub 150, or worse. It might very easily take them more than one GE to become competitive again, so whoever takes the job on after they lose the next one may well be signing up to be booted out after losing again, like eg William Hague was.
It just sounds like a lot of thankless, potentially unrewarded graft, for someone who's never been keen on hard work, and will have more pleasant, lucrative and easy options available to him if he wants.
TLDR- he wont stick being Opposition leader in the wilderness when he could go on the US right wing after dinner and board circuits.