Yes, the bones could be causing the blood--and if the intestine is perforated, that's big trouble. But inflammation, parasites, or even cancer could be causing the blood... Regardless, he needs to see a vet as soon as possible to determine the cause. If you can get a sample of the stool to bring in, too, that would save time.
Good luck with your boy and please keep us posted with what the vet finds!
