Kitten peeing is the same place

Everytime my eight week old kitten comes upstairs he pees or poops by the back door. It seems like that is the first thing he wants to do when he comes upstairs. He has clean kitty litter in his room, which I have seen him use, but as soon as he comes upstairs he goes to the back door to pee or poop. It is starting to get a little annoying. His mother and sister use their kitty litter and do not pee or poop anywhere else around the house. If anyone has any ideas or suggestions, I would appreciate it.
Thanks

Well, for one thing you need to deodorize the area he eliminates in. He continues to go there because he detects the odor.

At 8 weeks old he is too little to remember where the cat box is and what it is. He needs to be in one room with his litter box for a week or so. He just keeps going where he smells the last time he went. I wouls also add another box if the house is fairly big.