Diarrhea in young Nubian

Hey! So I have a 3 month old buck, I’ve had my goats for about a month; everything was going fine, they eat horse quality grass hay, no grain, only banana peel, watermelon or strawberry at a treat and it’s only ever very occasional, they have a few trees that they have stripped down the bark but recently they haven’t had tree leaves because they already ate all the leaves on the bottom of the tree, so literally all they’re eating is hay. They’ve had ivermectin, they get loose minerals 24/7 and they’ve not over eating it, but about 2 days ago he started having like mud pies, not goat berries; his brother is doing just fine, they both have plenty of energy and appetite; just having very bad poop; I’m incredibly worried but my large animal vet won’t have us in until Thursday, and it’s only Tuesday. Please help with advice or anything! I already have pepto