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Here is some good information on rabbits and pooping problems. Many are caused by dietary deficiencies but they can also be caused bu internal parasites. Take a look at the site and see which description best fits your rabbits problems. To be safe you may want to take him in to a rabbit savvy vet to have them do a checkup just to make sure your diagnosis is correct. At his age it is better to be safe than sorry.
Hope this helps!