Birds are small and do not have much blood to begin with. Your bird needs to go to the vet immediately. This is an emergency. Birds often hide the fact that they are sick until the situation is very severe, so symptoms like this mean your bird is in danger.
Bloody stool is a typical warning sign for intestinal parasites or internal bleeding. In either case, there is no effective home remedy, and your best bet is to contact a vet immediately.
See a Vet immediately! While diet does cause color change, if you think its blood and not say a strawberry or the red pellets he eats, then getting to a vet is the only option and the best bet to keep the situation from turning fatal.
