Oh my, what utter rubbish, to get to Nong Khai from Khon Kaen, you need to travel on the very same highway, going the other way !
You could take the train if you are that worried about being checked, or let Leo go alone, I'm sure he can manage. And I already understood the criminal check was for Leo, hence the visa remark.
But now I still see you want to get to Lao illegally, which is utterly stupid, you are not a Lao, so you will go from being illegally in Thailand to being illegally in Lao. Granted, the Lao are not this stung up with the whole issue of overstaying than the Thai, but it might be a good idea to bite the bullet, secure Leo a visa, go to the Airport and fly back to Canada with him. All that will happen is you will pay 20K overstay fine and get blacklisted for 10 years. No big deal
Edit: I just realized, you might actually be thinking of crossing the border at Nong Khai normally, and expected to get a 10 year blacklist + 20K fine, as opposed to crossing the border illegally.
Did you inquire if that is even possible, it wouldn't surprise me if they send you back to Bangkok with a massive overstay. Quite possibly to the IDC. If I were you, I would have a ticket out to Canada ready just in case..