Steve & Hazel

I second this - my now husband accompanied me to Montreal and into the consulate and interview. He was even asked some casual polite questions
I'm astounded by the number of negative, know-it-all responses in this thread which are un-informed. I'm sure the OP's fiance knows how to cross the border by now and she was asking a very specific question which didn't get answered

That's very odd. For the K1 Visa interview in Canada, the petitioner is allowed to accompany the beneficiary to their interview. My fiancé came with me, the consulate never asked him to come. The packet 4 email that I was sent actually mentioned that a US citizen is allowed to accompany me. Of course, I brought my fiance.
He was able to come into the consulate with his enhanced DL. The lady checking never even asked for his birth certificate confirming his citizenship.
The interview turned out great! I was approved after a short, maybe 5 minutes total if interview time!

Actually, he got in with his enhanced drivers license.

It isn't "foolhardy" at all. Canadian and American citizens can cross each others' borders by land with an enhanced drivers license. Flying will require a passport.
He has an enhanced driver license, which he uses to cross the border. It's an alternative to a passport.

He doesn't need a passport to drive through the borders, he will only need it if he flies internationally. He's crossed the border by land many times with his enhance drivers license. Since we will be driving to Montreal from Toronto, crossing the border for him will not be an issue.
My question is if he can get into the consulate with his enhanced DL, since and birth certificate only.

Don't give up. Stay strong, we're all going through this together, and you are not alone!