Hi there- Ok- it sounds like you and your vet have definitely addressed possible flea problems. The reason I asked so many questions about that is that many cats are actually allergic to the saliva of the flea. These cats can have a tremendous itchy response to just one single flea bite. Since a cat who is allergic doesn't have to be literally infested to have this response, the evidence of that one flea can be impossible to find. I'm glad to hear you are using any flea product. You never know when one gets in the house, especially if you live in an area where they are a problem! :)
Another reason for intense itchiness in one area is a skin infection. Sometimes we never know the reason something will cause the itching to start. Most cats really dig at themselves if they have an itch, and with sharp nails, the skin is sometimes broken. Since the nails on cats aren't very clean (ie: in the litter box all the time), they can actually give themselves a skin infection. This can cause intense itching and the more they itch, the more they spread it. Especially is there are sores in that area, I would ask your vet for 3 things:
1)another steroid injection so he leaves it alone and allows it to heal
2)an antibiotic for the skin (marbofloxacin or clavamox are good ones)
3)have your vet put "soft paws" on his back feet so he won't damage his skin when he itches
You could also use benadryl at the same time which will provide mild itch relief but may make him a bit sleepy. If you can't afford another visit for the injection, you could ask your vet if you could just pick up the antibiotic and an oral steroid. Ask your vet to give you the correct dose of benadryl (diphenhydramine) to use, which you can purchase yourself. Get the tablets so you can cut them, and make sure you coat them well before giving to your cat because benadryl is bitter!!
As far as allergies go, that is really impossible to prove in a cat. Itching skin can be a sign of either food allergies, inhalant allergies, or contact allergies. I doubt this is a contact allergy since it is on his neck. It may not be an allergy at all. If it heals with the antibiotics and doesn't come back, you will know. An appropriate food trial must be a MINIMUM of 6-8 weeks on that food exclusively (no treats, other food, etc) for it to properly work. After this time frame, with no response, then it is ok to try a different food.
There are allergy tests that some vets run but they are horribly inaccurate and expensive.
I think a lot of the food allergies in cats are caused by all the grains (corn, soy, wheat, rice) that pet food companies add to increase the profit margins. Cats are strict meat eaters. The only plant material they eat in the wild is in the small tummies of the rodents they catch, and this has been pre-digested by the rodent. I am a big believer in the newer, grain-free foods on the market. Some of my favorite brands are Innova Evo, Wellness Core, Instinct, Before Grain, and Weruva. Most feline and internal medicine specialists are also recommending only canned food. Dry food has been recently linked to all sorts of health problems because of the high grain (carbohydrate content) and the lack of moisture. If you are interested in this theory, you can read more at www.catinfo.org. This site was authored by a vet and doesn't try to sell you anything. Share it with your vet too.
I hope this gives you some options. Hopefully he will be itch free in no time. If you accept this answer, I wil be able to continue helping you on this same topic. You don't need to pay more than once! :)