Not with my Bully's but my old Border Collie Shep had Colitis and regular bouts of bloody diarrhoea. We eventually stabilised him on a low protein food and his flare up lessened considerably.
Its not uncommon fir dogs experiencing lots of diarrhoea to end up having blood in it like this. I would suggest not feeding him for 24 hours to rest his stomach then introduce bland diet slowly. Hope he gets better soon
os make sure you give him Metronidazole with food...it can make him feel sick and even vomit. It's the best antibiotic fir intestinal infections but causes nausea on an empty stomach xx