He can try. If they run his ID, it will instantly show up that he has a DWAI conviction though, at which point he will be denied entry. If they had already asked the “have any of you been convicted of a crime” question and he didn’t say anything, he can be arrested. Otherwise they simply turn him around.
So he can chance it, and bring cab fare in case he needs a ride home after walking back to the US side from Canadian Customs.
My friend with the DWAI ended up going to Toronto just a few weeks ago with some friends. Not his car, he wasn’t driving, and they didn’t ask for everyones ID.