However, Dave will not use them no matter how much I try with him. He is constantly yowling at the back door, which I find upsetting, and trying to sneak in via the cat flap, and having to be turfed back out again.

I am feeding him every time I feed the house cats so he is well fed and watered. But how on Earth can I tempt him into the shelters? Any one ever had to do that. Or should I wait until he decides to use them himself?
I'll be glad when I can take the poor mite home with me. That day cannot come quick enough.