Help with cat using outside toilet

[nacho]

Nanook is just over 1 year old now and has been going outside for the last 2 months.
We really want him to use the outside toilet but he keeps coming in to use his litter tray. Even for a pee.
All cats we have had in the past have managed to work it out,except Nanook. He's not the brightest of cats.
He hasn't even worked out how to push open doors yet!
Can anyone help.

 

[yayi]

I don't think Nanook's toilet behavior has anything to do with him being "bright". All my cats also go outside and they go indoors to use the litter box once in a while. The weather, the soil outside, the other stray cats affect their toilet preference. 

 Be patient. You can also help by maybe leaving the door open for easier access to the outdoors. I recently moved to a new place without a cat flap so I leave the back door slightly open for my cats.