Yellow urine isn’t an observandum. In fact, consistently clear urine with no chalky residue ever IS an observandum with rabbits. So you want to see some color and some chalkiness, because that’s a healthy bunny!
Yellow color doesn’t mean too much calcium. If you see gravel-like pee, ie pee that isn’t like pee at all but beige/gray and like fine wet concrete, that means bladder-sludge.
Here are two really good articles that you probably will feel better after reading:
red urine rabbit
normal vs sludge rabbit
As Azerane says, a litter box screen could be good if he gets pee on his tummy. Sometimes bunnies like to sit in their litter-box after having done their business, and some pee can get soaked up by the fur. Really fat rabbits can get pee and sticky poop on their tummies because they’re not capable of cleaning themselves very well, but your bun is young and probably not very fat.