The large soft droppings often indicate a condition called cecal dysbiosis. Rabbits produce a special dropping called cecal pellets. These are not regular feces, but are a rich source of beneficial bacteria. Rabbits must eat them to stay healthy. Dysbiosis usually occurs for one of two reasons: foods that are too rich in carbohydrates or an imbalance in the beneficial bacteria. You can read more about cecal dysbiosis here (scroll down):
Provide some beneficial bacteria (probiotics) for your rabbit. A brand called Bene-Bac is often available in pet stores and online. You can also use a human liquid formula, if it is nondairy. These are available in health food stores. Give some beneficial bacteria every day for a couple of weeks. After that, provide some twice a week.
Since your rabbit is also weak and losing weight, I suspect more than dysbiosis is going on. The probiotics will be good for your rabbit regardless of what is wrong. However, in addition to providing probiotics, you'll need to have him examined by a vet. Your rabbit may be suffering from a bacterial intestinal infection. Dental problems also often cause weight loss. You'll need a vet to diagnose exactly what is wrong, and provide proper treatment. Here is where you can find a rabbit vet:
You'll also want to review your rabbit's diet. Grass hay should be the base of the diet - he should have all of it he wants. Pellets should be limited to 1/4 cup per day for a 5 lb. rabbit. Gourmet rabbit mixes with seeds and dried fruit are too rich in sugars and refined carbohydrates. Fruits are also too sugary, and should be used only as rare treats. It would be better to feed greens, such as parsley, mint, clover and dandelions (from an unsprayed lawn only). Iceberg lettuce, however, isn't good. It has no nutrients in it and can lead to diarrhea. Introduce produce gradually. Too much, too soon leads to digestive upset. The following link will take you to a site with detailed information on rabbit feeding, including lists of the best produce, and what foods to avoid:
The most important thing to do now is make a vet appointment. After that, get some probiotics for your rabbit. If you have more questions, just let me know by clicking on REPLY. I hope your rabbit will quickly reach a full recovery.