Red urine is NORMAL in most cases. Many people have probably seen red/orange urine in the bottom of the cage and thought that the rabbit was bleeding. Some rabbits get it often, and some never have it. Red urine is basically when the urine contains large amounts of calcium oxalate, and forms on the cage floor. Certain types of feed often cause this such as alfalfa and Leucaena. It is often seen in the cooler months of the year.
If the rabbit is constantly showing red urine, he may have a kidney stone or a tumor in the bladder. There is no treatment for this and it could be very painful. Take them to a vet if the red urine persists over a long period of time, just to be safe.