Huge underdog is a huge understatement. But the CPC base and the CPC membership (and the CPC caucus) are three different animals.
I have three thoughts on why he would go for it as such a huge longshot
-genuine leader that thinks he can do good for the country, and is willing to sacrifice to try to make it happen
-riding might be remote, but a lot of ties to GTA/SW ON money, could be surprising amount of membership and donor support
-visibility for future run (either CPC or new Red Tory/PC wing if the party tears itself apart)
But agreed, between being pro-choice and anti-pseudo legal/ pseudo scientific rage mongering he's fighting uphill with a lot of points lost right up front.