Skin problem

Hi all, i'm a new member. Need some help and advice on my dog's skin problem. I have an english cocker spaniel around 3 and a half yrs old. He is suffering from many skin disorders like scratching and biting, hair loss, dandruff on body, rough and scaly like skin patches and sore patches.

I have visited vet many times but the skin problems still persist. Everytime i went to the vet, they will prescibe antibiotics, give jab and medicated shampoo.
Vet also recommend change food diet but still can't solve the problems.

Can anyone help me? Please . . Thxs a lot in advance .