hi babs, I am not a Vet however I will try to give you some advice for poor Beemer. you are right in thinking its the diet causing his hair loss and itching. it could be either of the foods, dry food isn't a good choice for any cat, its like eating at mcdonalds....lol esp for males as they run the risk of UTI's and crystals in their bladder...that being said EVO is one of the best dry foods out there, but all foods have different ingredients and dry has many preservatives too.
Hill's presc. diet isn't a good food, and its expensive. I will give you a link to a site from a Vet who has very good info on diabetes and what to feed, she also has a link on that site naming the best commercial canned foods.
There isn't any Vet on this site currently so if you need more help please visit us on the CAT FORUM...good luck to you both.