ER or Monday?

I have a 5m old bernedoodle with escalating diarrhea.
He went to daycare on Wednesday and was given peas, whipped cream, and carob covered strawberries, which are all new foods to him. When he had diarrhea a couple times on Thursday I wrote it off as too much new food or sensitivity/allergy to one of them. But it continued overnight into Friday, and now here we are on Saturday night.
I swapped him to chicken, rice, pumpkin this morning (also new, he's never had digestive upset like this before) and added FortiFlora probiotics. He's drinking fine & eating fine, but the diarrhea incidents are increasing in frequency, about 4x overnight and 5-6x during the day today. I tried to get a vet appointment this morning but they were booked, so we have one for Monday at 8:30.
I've peeked at his gums a few times, and they're pink and still moist, so he doesn't seem to be dehydrated. The way this is snowballing has me concerned though. Is there a point where this shifts to consideration for an ER visit instead of waiting for our appointment on Monday morning?
The stool appears a regular brown color, and it's smelly, but not notably bad. He's fully vaccinated, of course.