Is there any chance he got into anything toxic? Has the food or hay gotten wet or moldy somehow? Can you tell if he has prolapsed his rectum? In other words, part of it will be hanging out. This is one possibility
Coccidiosis, caused by a protozoan (one-celled organism) parasite, is a disease of the liver and/or intestinal tract. Rabbits become infected by eating food or consuming water contaminated with feces from an infected rabbit. Signs depend on whether the disease is localized within the liver (inappetence, diarrhea, death) or the intestinal tract (weight loss, soft to watery feces, mucus and/or blood in feces, soiled anal area, dehydration, increased thirst, possibly death). The relative severity of both types of infection depends upon the number of coccidia ingested, the age of the rabbit, the strength of its immune system, and other illness in the rabbit. Occasionally, the coccidia colonize the nasal passages, resulting in respiratory disease (nasal coccidiosis).
Coccidiosis may be treated with sulfa drugs. Emphasis must be placed on prevention (good husbandry and sanitation) of this disease in all rabbitries, since it can be difficult to eliminate in these situations.
Another possibility- are you sure you have a male and if not then it could be this (as the vulva and anus are very close in a female)
The most common tumor of domestic rabbits involves the uterine lining. In breeding rabbits, the early signs of this tumor involve decreased fertility, smaller litter sizes, abortions and stillbirths.
In pet rabbits, the most common clinical sign of a uterine tumor is intermittent bleeding from the vulva. This vulvar bleeding is often mistaken for blood in the urine. The volume of hemorrhage can be substantial and alarming. If bleeding is intermittent, the results of a urinalysis may be normal between bleeding episodes.
Though this type of tumor can spread to the lungs, spaying of affected does is strongly advised. Because this type of tumor is so common, all pet female rabbits should be spayed after 4 1/2 months of age to avoid difficulties with the reproductive tract later in life.
Either way your bunny needs to see a vet as soon as possible.