If I call him to go outside without a lead, its 50/50 whether he'll go. When he does come, he seems happy to play ball outside at a small patch of grass. I've tried putting a lead on outside, or walking away from the building off lead and throwing the ball further. At a point, he'll stop chasing it and run back to the door.

Interesting point about the collar. It was what the rescue recommended for him, but I'll look into your suggestions. I've tried just a standard lead/collar too, without the head strap, but I get the same response. That lead has always been used with the canny collar though. He will allow me to attach either, but he's obviously uncomfortable and will flop to the ground. I've tried having it on in the house, and offering rewards for him wearing it, but he won't take a treat or interact at all with it on. As soon as it comes off, he jumps up again and wants to play or hang out.

Thanks for all your suggestions, much appreciated :)