Skin problems - over grooming and scabs

Hi there We have a one year old rescue cat. We had him in December - no known medical needs - however in April he started getting small patches of no fur which would then turn into a raw sore (quite quickly). He also licks and bites a lot. The vets prescribed steroid injections, then steroid tablets - this has helped a little. He went off his food a lot in the last few months but is eating again now. He is still licking and biting a lot of the time and his fur, especially head, neck and legs, is quite thin. In the photo below, you can see a small white patch - the sore was also a small white patch until today. He goes outside when he wants, we live in an urban area but a relatively quiet one, with a few other cats. The vet felt it was some sort of dermatitis. Any other thoughts? We have insurance - the medication they are suggesting is around £120 a month, but I worry long term steroids are not good for him.