It’s a bit difficult to see if it really is thick and cakey of a worrisome nature, or if it is within the scope of normal. Here are some good pix of normal pee vs cakes urine:

http://www.medirabbit.com/EN/Uro_gen_diseases/generalities/Sludge.htm

You will have to keep a close eye of course. Encourage water intake by serving him wet greens and “watery” greens, perhaps cucumber, zucchini and small pieces of tomato. That would not add too much sugar to his diet. Start with small pieces unless he’s had those vegs before.

Some people have luck with some unsweetened cranberry juice added to the drinking water, to “flush out” sludge. Not all buns will drink water that has something added to it though, so always offer an alternative bowl of plain water.

agree it’s a bit hard to tell. What greens has he been getting? Have there been any less-sludgy looking pees as well?

Hmm… I wonder if this has something to do with the poor condition he is in. He could be dehydrated. I agree with the suggestions above.