Why Does My Kitten Keep Peeing In My Older Cats Litter Box?

My kitten keeps peeing in my other cats litter box. I will put the kitty litter down and he goes to his own box. When I come back after an hour or so, there is poop on the floor of his own cat box too! It’s not because he doesn’t like other cat’s boxes -he seems very happy when i leave him alone for a while- it must be where he pees that causes them to smell it? Any advice would be greatly appreciated! Thanks