I'm so sorry to hear about Mr Santa's grunting and on and off diarrhea over the past couple of days.
Is he eating normally?
I know his stools are loose on and off but is the amount of stool more or less than usual?
Grunting noises indicate pain in a bunny. This may be due to mouth pain secondary to sharp tooth edges with abnormal tooth wear, or abdominal pain or nausea.
His loose stools may be due to improper chewing (because his teeth hurt) or might be due to a gastrointestinal obstruction (hairball) or abnormal motility leading to changes in gut flora and increased gas.
Showing obvious signs of pain in a bunny indicates there is a serious problem. Bunnies are a prey species and will hide their pain until it is bad enough that they can no longer do so. If they don't chew properly, don't eat constantly, or don't take in enough fiber their gut motility slows down tremendously. Once a bunny's intestines and stomach aren't moving properly they can experience abdominal pain which can decrease their appetite further. Poor mobility can also cause bacterial overgrowth which inhibits normal digestion and increases painful gas accumulation, which further suppresses the appetite. This can also cause loose stools
There are several things that can cause loose stools and pain in a bunny his age. Tooth problems and abnormal wear of molars can make it painful to eat. A mass in the abdomen, unfortunately abscesses are common in bunnies too, or an obstruction from eating something non-digestible he was chewing on.
Ideally your bunny would be seen by an experienced rabbit veterinarian as soon as possible. Here is a link to help you find one in the area you are in now in case she gets sicker and cannot wait until tomorrow: http://www.rabbit.org/vets/vets.html
If he isn't drinking well and has loose stools he needs subcutaneous or intravenous fluids given daily as if he is dehydrated that will only make him sicker.
While you are waiting to have him seen I recommend keeping him confined, and force feeding him if he stops eating with a mixture of ground pellets, pedialyte and vegetable baby foods like green beans, squash or carrots every few hours to try and stimulate gut motility. If you can find Oxbow critical care diet that is even better for force feeding. He should be force fed frequently, every 3 hours during the day if you are able. If you cannot find Oxbow critical care your veterinarian should have this or can order it for you. If you are unable to get this in the meantime continue using a mix of ground up pellets, no sugar pedialyte and veggie baby foods in a syringe.
Make sure he has constant access to timothy or mixed grass hay. He needs fiber to restore normal motility.
Ideally he would also be placed on a probiotic to restore his normal gut bacterial population. Here is an example of a rabbit probiotic: http://www.nutramaxlabs.com/vet/products/Proviable-rb.aspx
I also recommend he be medicated with Reglan (metoclopramide) every 6 to 8 hours to try and restore normal gut motility. Your veterinarian can prescribe this.
Best of luck with your bunny. Let me know if you have any further questions.