He likely isn't having diarrhea because there is nothing TO diarrhea.
Can your partner check for dehydration? Tacky gums, the "scruff snapback" test. I would also feel his ears and see if they feel unusually warm/hot to the touch (fever).
Dehydration is my main concern. Secondary to that is a missed infection, which then also leads to ketones concern. Have you checked ketones recently?
I would try to assist feed (and water) at this point if he won't on his own. But if he has the signs of dehydration I mentioned above and/or ketones in urine I would take him in. Or if he starts acting very lethargic/off.
You can only give Cerenia once a day.
The ER vet would likely run bloodwork (probably similar to what your vet has already done but hard to know for sure, like CBC, chem panels), check for dehydration, check urine, give fluids, possibly prescribe metronidazole like Suzanne said (it's really good for GI infections). Plus they'll probably try to oversell you some stuff like FortiFlora (don't do it, many better probiotics out there for less money).