Might be a nerve in his neck or back being pinched. Cuddles couldn't walk, or run, a straight line when it happened to her. I'd check out everything else but if nothing is found, do you know of a good canine chiropractor? Can recall watching a weanling thoroughbred galloping and so busy watching the weanlings coming behind her that she ran into a gate. That jinked her neck and she held her head tilted afterwards.
The most dogs I've had, chico2, was 12, so no wonder the place is feeling empty now we're down to 8. Thankfully they weren't dogs the size of Hazel's Setters.