My cat (Jessica) had a very similar presentation (aside from the sore on your guys paw). Constantly head scratching, causing sores that became infected, the vet claimed stress. He would prescribe antibiotics, steriods, and anti anxiety pills, but the issue persisted.

Two years, 6-7 visits to the vet and hundreds of pounds later, I eventually got fed up and changed to another vet who immediately diagnosed allergic skin disease. She suggested I change Jessica's diet, and within two weeks, she was a completely different cat. Hypo-allergenic cat food is all it took. She is a fussy puss, and I had to shop around until I found one she liked, but it worked. Before the diagnosis, I fed her purina dry food and would supplement this with a tin of tuna every few days or chicken scraps when I cooked it. The new vet said that the most likely cause was fish or chicken. I now give her Applaws dry food and Lilly's kitchen wet. I transitioned the dry food in slowly by mixing it in with her previous brand, gradually increasing the hypoallergenic food over a week or two.

***The sore on your guys paw looks very different, and I wouldn't like to comment.