Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm so sorry to hear about your fellow not really wanting to eat and lethargy since the weekend, and I'd like to help.
If he isn't eating and drinking then he won't make much or any waste, and that is concerning too.
Is he grinding his teeth at all? Grinding his teeth can be due to pain from sharp tooth edges, abdominal pain, or nausea.
A loss of appetite in a bunny is a serious problem. If they don't eat constantly or 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. I am concerned that is where your bunny is now.
There are several things that can decrease appetite in a bunny. Tooth problems and abnormal wear of molars can make it painful to eat. A mass in the abdomen, unfortunately tumors are common in older bunnies, or a respiratory infection that makes it difficult to breathe are other possible causes.
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 he gets sicker and cannot wait until tomorrow: http://www.rabbit.org/vets/vets.html
If he isn't drinking 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 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, 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. I hope things work out for him. Let me know if you have any further questions.