Just another person worried about the US/CAN border:

I just went through the same thing, pretty much exactly, except for I needed to fly home. Did a lot of googling, and it seems like for now at least you should be able to cross the border on land with no problems (I also had no problems flying), but things can change day to day.

There will probably be some warning in the event that a total border closure is imminent, but there are no guarantees. I was planning to leave yesterday originally, but since my college announced it would be all online, we were thinking about spending an extra week together until the border started closing, and there was a lot of stress in regard to making sure I wouldn't get stuck there for months. I'm home now but I feel so robbed : /

What I keep telling myself is that if I'd gotten sick there things would have been awful, and I would have had no health coverage, so leaving ASAP made sense. Plus his housemates would have been very unhappy if I had to stay indefinitely. Even though it makes logical sense, emotionally it hurts pretty bad having to give up the time together not knowing when all of this craziness will be over.

Talk to him, and maybe his roommate and see what's best for you. From the looks of things staying a little longer is probably safe, as long as you keep your eye on any announcements and you're ready to leave if you have to/have discussed not leaving in the event that the border is going to get locked completely.