What it sounds like you are describing is colitis or inflammation of the large bowel as your vet previously suspected. The signs can be very intermittent can vary in severity, from normal stools to mushy to very soft, all even within one day at times. Seeing mucus or blood is not unusual as well. There is a sense of urgency to move their bowels and many of the pets will strain when having a BM, which to the owner can almost look like they are constipated. Many pets with colitis have GI discomfort and some will even vomit.
Colitis can have many causes including intestinal parasites, dietary indiscretion, food allergies or intolerances, metabolic disturbances from internal organ malfunctions, viruses, toxins, cancer or even stress. It is possible that even if he did not receive any human food, just the stress and change in routine of having visitors this weekend set it off. The drooling and gagging are a sign of nausea. If it continues, your veterinarian will want to know the history of his signs and will need to perform a physical exam to help rule out the other causes mentioned. A stool sample analysis is a very important part of getting to the bottom of this as intestinal parasites are one of the most common causes.
Until you can get in contact with your vet, I would fast your dog to give his GI system a rest. Since he is an adult dog, a 12-24 hr fast will not hurt him and will let his gut calm down. Continue to offer small amounts of water at a time. When you do begin feeding him again, you will want to offer frequent, small meals of something very digestible for him as you have been. These would include items like boiled hamburger or chicken and rice or pasta. No sauces or gravies, but a little non greasy broth would be OK. If the stools improve, I would continue that for a few days and then slowly wean him back onto his normal food and feeding schedule. If he won't eat, or if the diarrhea of nausea signs continue, or he just can't seem to get comfortable, I would have him examined.
When they have been going on chronically, these GI disturbances can be frustrating to diagnose and treat and sometimes we don't even find the cause, but concentrate on controlling the symptoms so our pets are comfortable. In his case, since it appears to be an acute flare up, there is no reason to think that it won't resolve completely. Make sure to get in touch with your vet if he is not improving so they can get him treated appropriately and verify that there is nothing else going on to be causing these signs.
Please let me know if you have any further questions.