I just posted this on the other thread you replied on:
How is your pumpkin doing??
A food could possibly cause bloody stools if there is an allergy or intolerance but I think that it is usually accompanied by vomiting and/or diarrhea, as well.
I just did a Google and found this:
Dog Blood in Stool | Dog Health Blood in Stool
Hoping he is doing better....!!
Keep us posted!