
Don't attempt to feed him and make sure he has plenty of fluids.
Sudden onset of bloody stools - particularly blood in the quantity you describe sounds as though you'd be better to at least give your emergency vet a phone just now and see if they recommend bringing him in. Hopefully it will be something quite simple but equally it's better to act sooner rather than later as the symptoms are so sudden and sound quite severe.
Good luck and please post back how things go. Teri