Glad to be able to help.
And, thanks for the clarification about the drugs.
I can understand the difficulty you've having in trying to decide if he's vomiting or coughing. It sounds more like a cough to me but, again, I can't entirely know for sure.
It's possible that his intermittent diarrhea issues are secondary to internal parasites which is why your vet dispensed Fenbendazole (an excellent wormer, by the way). It's also possible that he's picked up a viral or bacterial infection or even IBD might explain his symptoms or stress from the groomers.
But, regardless, I do have a few suggestions which, hopefully, will help:
1. Cerenia doesn't need to be given on an empty stomach. It's a very potent anti-nausea medication and I often use it for my vomiting patients, although not ones with loose stool issues. It also has anti-inflammatory properties which can be quite helpful.
Most dogs tolerate this drug very well but diarrhea is listed as one of the possible side effects. In a situation like this, it may be difficult to know if the diarrhea is secondary to a drug reaction or not.
I might continue to monitor the situation for now to determine if he's actually vomiting (if he has another episode) or if he's coughing. If the former, then I'd give it but if the latter, I wouldn't.
2. A bland diet such as you mentioned wouldn't be a bad idea. My ratio is 1 part boiled, skinless, chicken breast (or lean hamburg) to 4 parts 20-minute boiled white rice, small amounts through the day.
The EN is a good food as well but I might consider the bland diet.
However, I'd let things settle down for now. I wouldn't offer him any food for 8-12 hours from now. I'd let him drink small amount of water (or pedialyte) but only small amounts at one time.
3. Continue the probiotics.
4. If it's clear that he's not vomiting and he's eating, then dose him with Fenbendazole. I assume you have three days worth which is the usual amount dispensed. If he has internal parasites, his diarrhea issues may not totally resolve until he's wormed.
You don't have to give this drug with food but I typically recommend that you do.
I hope this helps. Deb