White/paste-y urine is most likely having an excess of calcium. Does it dry, leaving a white powder behind? I’m guessing yes? White urine is caused by excess calcium being exited through the rabbit because it’s not needed. The occasional white pee is not bad, but high frequency is a bad sign, as it means your rabbit is getting too much calcium and it could be building up in your rabbit’s digestive system, in addition to some exiting. The calcium can build up and literally form a calcium stone that can be as big as a quarter, which blocks everything in your rabbit, causing drastic consequences. Rather than combating this by increasing liquid intake, it should be by lowering calcium intake, which you have already mentioned.
A review of his entire diet would be helpful to everyone!
1) How old is he?
2) How big are his pellet portions, per day?
3) What type of pellets?
4) What type of hay?
— Veggies can be a source of high calcium, but alfalfa is a large calcium contributor and usually the biggest culprit when rabbits are younger because it’s so dense in nutrients!
Also as a general note, has your new bunny seen a rabbit-experienced vet yet to make sure he’s healthy and doesn’t have any pre-existing conditions?
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.