Thanks for the additional information and answers to my questions. That's great that he's not otherwise acting ill.
Blood can always be concerning but unless there's a constant profuse amount of it, then it usually is secondary to intestinal irritation and nothing more serious than this. When large amounts are present, though, HGE (Hemorrhagic Gastroenteritis) is a concern and can be quite serious and requires hospitalization in most cases....although these patients are usually acting ill.
Metronidazole is an excellent drug for this situation and one I would have prescribed; the dose is about what I would have used as well.
And, while I haven't used Diphenoxylate for quite a while for my diarrhea patients, this is about the dose that I'd have suggested as well.
There are actually a number of different causes for diarrhea/blood in a dog which range from viral/bacterial infections to internal parasites to dietary indiscretion to Inflammatory Bowel Disease just to name a few examples.
Regardless of the cause, I assume your vet also recommended a bland diet such as boiled, skinless, boneless chicken breast and 20-minute white rice, several times during the day.
I also suggest a probiotic such as Forti Flora which may help to heal the intestines a little faster. This product is often available at local pet or grain stores.
As long as Toby isn't getting worse, then I'd give the drugs a few days to be effective; however, if he becomes lethargic and/or starts to vomit, then a recheck may be prudent.
I hope this helps. Deb