Maybe he's allergic to the shampoo?? I have never bathed my cat (she'd kill me if I tried), she keeps herself clean enough. From what I've read it's best not to bathe them unless necessary (gets into something she shouldn't, gets something on her you don't want her licking off, or is older and not able to clean themselves properly anymore, etc...). I could be wrong but that's my two cents worth!!
If the steroid injection stopped your cats allergy for a year it sounds as if you already have the answer to your question. You must take him to the vet for another cortisone injection. Your vet could also give you Revolution, Frontline or equivalent topical flea and tick medicine.
Since you are unable to take your cat to the vet since you have no car, find a house call vet in your area.
the vet says my cat ha s an allergy. NO fleas. needs cortisone shot. will contact in-home vet care service. THANK YOU for prompt response.