Why is my staffy X Lab behaving oddly? Is it worms or something else?

He stands still with stiff hindquarters and turns his head from side to side as if trying to get at his rump. He also licks the air when he does this and groans. I have been to the vet and he cleared his glands, yet the behaviour continued. Worms have been suggested because he is unable to 'scoot' due to his short stature and so does this behaviour instead. There is no evidence in his stools and I have wormed him. The behaviour eased off for 3 weeks and is back again. Our other dog shows no symptoms of worms.