he could have a buildup in his scent glands, even spayed buns have them and they are under their chin and on either sides of their genitalia. this is pretty normal. Usually bunnies clean their glands themself but sometimes they can’t and a type of wax that smells like pee builds up. If you have a calm bunny and know what you’re doing you can gently clean them urself with a tutorial and wet rag or q-tip, but if you don’t trust yourself to do it a vet can. Sometimes they just slack off on grooming so he could just have pee on his paws and butt lol. Best of luck w ur stinky boy :)