Possible Giardia in puppy?

So I have a 4 month old mastiff mix (mastiff, Presa canario, bouvier mix), he just got his last round of puppy vaccines on Monday and weighed in at a whopping 15.4kg. On Tuesday he had his monthly nexgard.

Now, Thursday I noticed his poop was a little softer than normal, and Friday night he had some pretty bad diarrhea with lots of mucus in it.
Today I gave him some plain rice and boiled chicken to try and help soothe his stomach, but as one does I panic googled everything, and the only thing I found that “fits” is giardia. So while I anxiously wait for our vet to open on Monday, any advice would be greatly appreciated and welcomed!