Agree with Lankeela. He needs urgent vetinary treatment. There are several reasons that he could be passing blood and only investigations by a vet can give you the correct diagnosis. But from the bowel could be very serious, so don't delay. Hopefully, it is something that can be treated quickly and successfully. Don't take any chances.
Yep agree, vet job. Could be Colitis, but definitely needs veterinary treatment as there are number of diseases where blood in stools is a common symptom.