Hello, I'm Dr. Deb and will do my best to help you today.
I can understand your concern for your dog but in most cases, the presence of blood is secondary to intestinal irritation and nothing more serious than this. While it can be alarming, it doesn't necessarily have to be concerning unless there's a profuse amount of it.
The fact that he's otherwise acting like his normal self indicates that this isn't an ER type of situation...not yet anyway.
Possible explanations for the blood certainly could include ingestion of some foreign material which is doing some damage as it passes through his intestinal tract; other possibilities include viral or bacterial infections or possibly internal parasites.
If this were my patient, I'd suggest a bland diet of boiled, skinless, boneless chicken breast and boiled, 20-minute white rice, smaller meals throughout the day.
I might also consider a probiotic such as Forti Flora which may be available at local pet or grain stores.
I probably wouldn't give him any over the counter Pepto Bismol at this point but wait to see if this problem resolves itself pretty quickly.
I don't typically take an x-ray for acute problems of this nature although every vet practices differently. Paper won't show up on an x-ray although wire, bones or metal certainly would.
I hope this helps to reassure you about him. Deb