[Ladylavender]

I don't think you have too much to worry about. If he is already going outside and NOT in his litter box, then he is already acclimated to the idea of going outside and not having the litter box there won't be a problem.

However, if you are looking to have him outside trained all the time even when he is at home, then you can train him to ask to go outside, but do as YorkieKids has suggested. Either train him to go out every hour on the hour, so he gets the gist that that is where he is expected to go from now on and eventually will ask you to go out or put the bells on the door and train him to ring the bells every time you go outside.

Or.... I would suggest putting in a doggie door, if his access to the yard is safe (i.e. fenced in) A doggie door will do allow him access to outside anytime he wants, but without having to keep the door open and letting cold air come in.