A cat that's losing furr and going crazy itching

Hey guys my cat BJ lost a lot of fur around his face and neck also had a few weeping wounds a few months ago I took him to the vet they gave him a needle and some antibiotics did absolutely nothing got me thinking of what i had changed in his life style at the time and the only thing that come to mind was his food and laundry powder so i went back to his old food and old laundry powder at the same time and at the end of the week i noticed he wasn't scratching and his wounds were clearing up and all his fur grew back all of a sudden a week ago ive noticed hes been scratching again and losing his hair and weeping wounds only this time it's a lot worse I haven't changed his food but have changed laundry powder whats the chances of it being that? HELP ME!!!!! it's making me feel so so so bad I'm trying my hardest to figure out what it is so he doesn't go insane lol

                                                                          thanks in advance guys

also forgot to add that hes also lost a bit of hair around his bum and when i fleed him i also wormed him the same day 

Could it be a flea allergy?