If the urine is actually thickened and gel like, creamy texture(not just the color), or pasty and/or gritty, then that indicates a condition called bladder sludge. A UTI will also occur concurrently with bladder sludge, so he likely would have that as well. It can make the urine look a beige or a brown color. There may also be spots of blood in the urine.
With bladder sludge and a UTI, signs could be lack of appetite, lethargy, straining to urinate, behavior indicating discomfort(such as unusual grumpiness, thumping), tooth grinding, other unusual changes of behavior, pee accidents, thickened urine, dribbling urine, beige or brown colored urine, bright red urine, spots of blood in the urine, urine soaked bottom.
http://wabbitwiki.com/wiki/Bladder_sludge_and_stones
http://wabbitwiki.com/wiki/Urinary_tract_infection
If there are signs of either a UTI or bladder sludge, or both, I would make an appointment with a knowledgeable rabbit vet right away. For my bladder sludge prone rabbits, I found I had to reduce calcium rich foods in their diet, as well as making sure they have lots of hydration to help flush the calcium sediment out(sometimes even requiring administration of sub q fluids), and are active hopping up and down from things to help keep the calcium sediment from settling in the bladder so that it is more easily excreted.
https://rabbit.org/lowering-blood-calcium/
https://rabbit.org/vet-listings/
If the urine is a thin normal consistency but just creamy colored, then it would mean he is getting an excess of calcium in his diet, but he is not prone to it creating bladder sludge(only some rabbits are prone to this). And for that I would just reduce some of the high calcium foods in the diet so as to not overwork the kidneys. If the urine is beige or brown colored, it's possible he has a UTI. And if he is having accidents outside of the litter box, this could be why and not just from hormonal territorial marking.