Oh no - poor Couver
!! I'm so sorry to hear he's feeling poorly. And how frustrating to not really know why.
Hopefully, this is just some sort of passing gastritis...but it is concerning that the poos were so dark...that can indicate blood higher up in the system, but it doesn't mean that for sure. I too would be wondering about pancreatitis, IBD/IBS. One thing about that darn Giardia is that it's notorious for hiding under scope...when Marcel had it as a puppy, they had to test THREE times before they could actually find it! So I wouldn't quite totally cross that off your list yet...keep it in the back of your mind maybe.
Maybe try calling the vet and asking if there is something they could prescribe for his tummy rumbles..?
Hugs to you both!