A trip to the vet is the first priority. All new buns needs to have a baseline exam, and especially since he’s already showing symptoms of something being wrong. Coccidia is extremely common in baby buns. It is easily treatable if caught in time, but it is fatal if left too long.
Here is an article about “Rabbit Poop” from rabbit expert, Dr. Dana Krempels: http://www.bio.miami.edu/hare/poop.html
There is a lot of info about runny poops in baby buns, but here is a small snippet. Click the link above for the full article.
Runny Stool: When Things Go Wrong
True diarrhea (unformed, watery feces) is relatively rare in adult rabbits. It can be caused by intestinal parasites, such as coccidia (Eimeria spp.), roundworms, tapeworms, or anything that inflames the intestinal lining. Your rabbit-experienced veterinarian will be able to examine a fecal sample (you should probably provide the vet with a fresh sample both fecal and cecal pellets, if possible) to examine them for signs of parasitic infection. More often, true diarrhea is seen in infant and juvenile rabbits.
Diarrhea in Baby Rabbits: A Life-Threatening Emergency
One of the most common signs associated with sudden death in baby rabbits is diarrhea, which can appear very quickly and kill in a matter of hours. Thus, it is of vital importance that any baby rabbit showing signs of lethargy, inappetence (not wanting to eat), or runny stool be taken immediately to an experienced rabbit veterinarian for emergency, life-saving treatment. Delaying even for an hour or two can mean the difference between life and death.
This is not something you can take a “wait and see” approach with. If your bunny was older (an adult over a year old), improper diet would likely be the culprit. But at such a young age, chances are it is an intestinal parasite. Do you have a rabbit vet yet? If not, we may be able to help direct you to some resources in your area.