hmm... at that age a lot of things can go wrong. If you haven't changed his diet in the last few years, I wouldn't expect it to be that. Are the pellets timothy hay or alfalfa? Alfalfa isn't reccomended for adult bunnies because it has a lot of extra calcium (and protein) that they don't need. Extra calcium can come out in the urine a lot. But it would probably be white and "sludgy."
I'd take him to a good rabbit vet soon, because my guess is that it's a urinary tract infection, or a parasitic infection called e cuniculi (EC). Make sure to ask the vet if she thinks it's e cuniculi. I say this because EC is a parasite that starts its disease progression by stressing the kidneys, leading to increased urine output. It also has neurological aspects, the first sign usually being dragging the hind legs. It eventually progresses into other neurological problems, including head tilt and inflammation of the brain.
I'm not trying to scare you here, but it's better to see what the vet says than not. EC can be treated in some cases, especially if you catch it early. I'm pretty sure the drug they give is called Panacur. Look for the member named Cheryl 's posts about her bunny Pippi, whom she lost to EC not long ago.
Most bunnies have been exposed to EC at some time in their life, but it is more common to develop into a full-fledged disease when they get older. It can lie dormant for years. For instance, my bf's bunny tested positive for EC at the vet, even though he doesn't have any symptoms of the disease. It just means we need to watch for symptoms like what you're describing when he gets older.
I really don't want to be a downer, but I want to stress how important it is to get Rocky checked out by a good vet. Maybe (hopefully) it's just a urinary tract infection or he needs a change in his pellet type.
Good luck to you two--:clover::rabbithop
ETA: for grammar, and to mention that there's a listing of good bunny vets in the library section. if the vet hasn't heard of EC, they're not a good bunny vet!
