Bloody diarrhea could have a lot of causes. The first and most likely (if he has already been vaccinated) is a parasite problem. If you have not seen the vet recently for a fecal exam, I would suggest that you go ahead and do so.
My other fear is that he ate something strange on his "trip." Rat poison and other toxins can cause unexplainable bleeding.
If he is of a normal activity level and eating well, you can change his diet over to a bland diet of only boiled chicken (no bones, no skin) and white rice and keep a close eye on him.
Then see the vet if this does not improve.
Let me know if you still have questions.