Blood in the stool can be caused from a variety of things. At his young age the most likely cause would be either a parasite such as a tapeworm or ingestion of something such as a toy or rodent/bird, etc. You can deworm your cat with a dewormer containing an active ingredient of praziquantel which will eliminate tapeworms and see if that clears it up. If your cat develops other symptoms such as inappetence or vomiting or diarrhea I would go ahead and take him in for a checkup to see what is going on. If you think there is any chance your cat may have ingested some rat or mouse poison; this too, is a cause of blood in the stool and you would need to have him seen by a DVM immediately.