Having driven across the US/Canada border multiple times, egress from the US isn’t controlled - or at least, it wasn’t when I went through last time. But I’m also a US Citizen and may not have been subjected to the customs queues in ways that non-US passport holders are - idk, it was a few years ago.

I’d think the same, though it sounds like the US border guards might try to stop him ever reaching the Canadian side.

Right now there are only a handful of border crossings from the US to Canada that have border guards challenging Americans leaving the USA, and all of these are new under trump in the last 6 months. The usual place USA border guards challenge is on entering the USA.

If he's gonna sneak across, he's gonna need to do it before the snows hit Saskatchewan. It's a long walk and it's very cold and many people die even when their home country isn't trying to make that happen.

On the Canuck side there's usually po-po waiting with blankets and cookies and a warm car. Of course he'll stay At The Pleasure, for a while, until his case comes up, but we're not barbarians.

I would not be shocked if that policy changed at some point in the relatively near future.

This. They actually have to report any American who tries to seek asylum to the American Government

Fly from Canada to Europe and try to get asylum there.