Hello Dee and Joe, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern about Dexter and to hear that he's been diagnosed with GME but glad that he's being adequately treated for this problem.
To address your questions as you've asked them:
1. Since Dexter is otherwise acting his normal self and there's not a profuse amount of blood, I wouldn't consider this to be an ER situation..not yet anyway.
Small amounts of blood often reflect irritation to the colon and nothing more serious than this. If he were going to have significant reactions to his medication, I would have expected to have seen them much sooner....not weeks after being on these drugs. I suppose it's always possible for him to develop side effects from the drugs he's taking but in my experience, it seems more likely that something else is going on. Possible explanations for colitis include viral/bacterial infections, dietary indiscretion, Inflammatory bowel disease, stress, intestinal parastes...and this is only a partial list.
2. If his stools continue to be loose with or without blood, then a visit to the local vet may be prudent. I'd run a fecal sample to check for intestinal parasites since giardia, whipworms or hookworms could be responsible for this problem.
3. That's a tougher question to address since I'm not certain that the pred is related to the blood/looser stool issue. If this were my case, I'd probably advise giving him his morning dose since this isn't a huge amount of pred for a dog this size. ...it's certainly not an immunosuppressive dose. However, if you wanted to give him 1.25 mg instead, this might be preferable to not giving him any drug at all.
4. The gastric medications won't be harmful but tend to be more effective if there's gastric ulceration or bleeding. If this were the case, then the stools would be a black, tarry color (reflecting digested blood) rather than a bright red color (indicating a large intestine issue).
I don't typically recommend Famotidine for diarrhea issues so I don't see the advantage of giving it.
5. I'd feed him a bland diet of boiled chicken breast (or lean hamburg) with 20-minute white rice, multiple meals through the day.
I'd also start a good canine probiotic such as Forti Flora which may be available at local pet or grain stores.
I wouldn't give him any otc products such as Pepto Bismol because they contain salicylates, the active ingredient in Aspirin. Since he's taking pred, this drug would be contraindicated.
I hope this helps and that his stools firm up soon. Regards, Deb