Cat Flea Allergy, how to stop the itch? -Jan?

[Jan]

My poor cat Pie is allergic to fleas. We found this out b/c we took him to the vet for his OCD itching and scratching...leaving sores on his back near his tail. The vet gave him an allergy shot to help with the itching, and told us to start putting him on some flea meds in a 2-3 weeks. Welp, we waited about 3 weeks and he has those sores again and is itching. He's so miserable. I called the vet to get another shot and start the flea meds, but they said they can only give those shots every 6 weeks or more, and that if I start the medicine he'll eventually stop itching. That's not what I wanted to hear....the scabs on Pie's back look painful and he never stops itching. Plus, he's cranky b/c he feels so miserable..we thought he was just moody but noticed he was fine when he wasn't itchy. How do I give him relief untill this reaction goes away? Will a lick or two of a children's antihistamine work? I would think a topical hyrdocortisone would be licked off and eaten..that's not good Help!
the flea medicine I'm talking about is regular preventative, like Advantage.